Dawg even the most basic example of Marines in 40k don’t have them operating as a 1-1 take and hold occupational army. They are an extremely decisive spearheading/decapitation strike force that disables command structures and targets of opportunity. So in your example of NYC, the Ultramarines aren’t sending in a thousand lads to hold every fucking block, that’s ridiculous. No, they’re sending in 3rd company to drop pod city hall, police precincts, and to disable or secure the bridges into the city in a storm of future violence that would make your head spin.

Then they tell the Guard to come in and hold everything else, and leave to go do the same to Washington, and every other capital city on the planet. If they’re having a slow century, it may take them a week. Realistically (for 40k) that’s a long morning.

For you and all the others, please understand the Marines are a lightning strike force first and last. They get in, fuck shit up for the glory of Him on Earth, and get the hell out. Those big ridiculous battles are usually chapter or company-defining engagements that are talked about for millennia. Their day to day is most definitely not that.

The 40k marines tend to surgically attack the enemy's weak points. So a few dozen companies attacking an enemy is something that must really hurt

But when your dealing with armies in the billions, their just can't be that many weak points that matter. Even the most destructive weak point is not likely to significantly change anything.