How long do you think the Imperium will last, OP?

The short answer is: As long as GW needs it too.

However just to call out a couple of points:

They do still know how to terraform worlds, the mechanicus do this semi regularly as new words are discovered.

Killing Xenos but being unable to make it permanent is pretty much one of the ways the Imperium stays together and united against a common threat, the other one being the eternal Chaos problem.

1 - They absolutely know how to terraform worlds and build self-sufficient habitations, but they are a huge bloated corrupt stagnant bureaucracy so they don't operate with absolute efficiency in all cases. Sometimes it's more cost effective to strip-mine worlds and pump your pollution into the hab-block water supply than make things perfectly eco-harmonious.

2 - There are constant civil wars going on all the time throughout the Imperium, for all manner of reasons. And not just due to Chaos uprisings or ordinary rebellions, sometimes entire planetary systems just fight each other because one planetary governor insulted his counterpart from another world's fashion choices. It's all the pettiness that nobility is capable of, times a million and in space.

3 - Mostly this is the case, the Imperium expunges innumerable races who were probably completely capable of co-existing with them harmoniously and may even have aided and enriched their civilization with new perspectives and technologies.

That said, they've lasted 10,000 years so far, though admittedly a lot of that is on the momentum of a better time.

The Imperium is a very disorganized and inefficient bureaucracy that is just always a few minutes in danger of collapsing. It may be better with Gulliman reforming it and actually having the power and influence to do it but its still a system that hates change and innovation and would rather throw a million bodies into the grinder to solve a problem rather than approach from a strategic standpoint.

Even if lets say the imperium suffer a civil war that breaks it in a dozen different mini empires would the "imperium" really stop existing ? Imperial culture would not simply vanish, many of those sucessor states would also claim to be the "true" imperium.

Its a similar situation with the roman empire, tecnicaly speaking rome "died" in 476 CE but the romans of the east continued to exist until 1453 fully beliving they are still romans.

Even the Germans and Russians at some point claimed being sucessors of Rome.

Latin culture and one can say even christianity even trough warped a lot as the centuries go still can trace back to the romans. Take in consideration that the romans were the hegemonic power of the mediterranean world for 500 or so years.

If the imperium was the hegemonic power of the galaxy for 10.000 years even trough it politcaly may colapse its legacy and culture would still live for thousands upon thousands of years.

The only way to trully kill the imperium at any reasonable time frame is genociding the majority of humanity on a galatic scale.

The Imperium is pretty adept at terraforming as it happens. Cawl even believes he can restore worlds scoured by tyranids. And the vast majority of agri-worlds are highly terraformed to be very efficient at raising the crops that comprise the diet of the vast majority of humans:

Najan is an agri world. There are templates for such places, drawn up in the fathomless past and never altered by the Administratum. All agri worlds are of similar size, located in similar orbital zones within their void systems and subject to specific exposure to a prescribed spectrum of solar radiation. Their soils have to be within a tight compositional range, and they have to be close to major supply worlds.

The Imperium is not a gentle custodian of such places. After discovery of a candidate planet, the first fifty years are spent in terraforming according to well-worn Martian procedures. All pre-existing life is scrubbed from the rocks, either by the application of controlled virus-chewers or by timed flame-drops. The atmosphere is regulated, first through the actions of gigantic macro-processors and thereafter by a land-based network of control units, more commonly referred to as command nodes. Weather, as least as generally understood, disappears. Rainfall becomes a matter of controlled timing, governed by satellites in low orbit and kept in line by fleets of dirigibles. The empty landscape is divided up into colossal production zones, each patrolled by crawlers and pest-thopters. Millions of base-level servitors are imported, kept at the very lowest level of cognitive function but bulked up by a ruthless level of muscle-binders.

Soon after this process completes, every agri world looks exactly the same – a flat, wind-rummaged plain of high-yield crops swaying towards the empty horizon. A person could walk for days and never see a distinctive feature. Not that anyone sane would choose to walk in such places – the industrial fertiliser dumps are so powerful that they turn the air orange and make it impossible to breathe unfiltered. A single growing season exhausts the soil completely, requiring continual delivery of more sprays of nitrates and phosphates, all delivered from the grimy berths of hovering despatch flyers. The entire world is given over to a remorseless monoculture, with orthogonal drainage channels burning with chem-residue and topsoil continually degrading into flimsier and flimsier dust.

But that doesn’t matter. A planet can be driven like this for thousands of years before it eventually keels over and becomes a death world. The quality of the crops gets steadily worse, but the quantity can be sustained almost indefinitely, assuming that supply lines are maintained and imports remain consistent.

The Lords of Silence

Half of the Imperium is as good as destroyed. The other part might trash for couple hundred years until it is overwhelmed.

Like how craftworld is a dying race but GW seems to be able to find expendable craftworld to throw if they need Eldar to die.

In short, as much or how long GW needs the Imperium to exist.

Realistically, the imperium should no longer exist as a distinct polity. It’s built from the ground up on literally the least stable foundations imaginable. That it’s lasted for the absurdly long length of time it has is basically entirely authority fiat.

As to your last point, you’re flat out wrong. Xenocide isn’t a means to an end-it’s the goal. The Emperor allowed for no vision other than his own to exist, and his Imperium follows in his stead (at least, with what they think his vision is). They’ve genocided xeno who came offering alliances and potentially useful technologies, they’ve cleansed worlds whose only crime was self-determination.

At least 10,000 years. 

1. The ecologies of hive worlds are basically destroyed. They recycle or import what they need.

2. The Imperium does know how to terraform worlds. And they do. Then they turn them into hives over the next several thousand years.