If they had the fatality rate I mentioned. You mean like the Devastation of Baal? Where the Blood Angels had to pull together as many of their successor chapters as possible against Hive Fleet Leviathan and were still almost wiped out entirely? Or Maybe the Battle of Macragge? Where the Ultramarines lost most of their navy and had multiple companies utterly decimated, and the first company destroyed, by Hive Fleet Behemoth? Or maybe you mean like the numerous chapters who were on the brink of extinction until the Primaris reinforcements arrived? Or, perhaps, even, we can look at Space Marine 2, the aforementioned video game, in which the majority of the second company is killed over the course of that one campaign?
There is a reason that the storage of gene seed is so important to the astartes. There is a reason new marines are always being trained and the vast bulk of battlefronts are taken up by billions and billions of guardsmen and human auxiliary forces. Yes, 1000 guys is not enough logistically, GW is bad with numbers, we know this. We also know that they're constantly getting fucked up and losing guys, so GW is, at least, somewhat aware of the back foot they should be on, and demonstrates that it is, in fact not enough now and then.
So, I'm sorry to say, it does not in fact work "like in the videogame." Astartes are extremely killable.
STILL waiting for that game where you play a Genestealer Cultist with a Rock Saw who systematically kills an entire Space Marine Chapter (heroically).
As much as I despise the “War of the Beast”, that series had the Imperial Fist chapter driven to extinction and it had to be reassembled by successor chapters. Space marines die a lot, but a million worlds of people procreating makes the difference.
