If this had been the climax to Fallout 4's main plotline I'm not sure anyone would be complaining. Well, they would, because Bethesda RPGs and complaining go together like power armour and fusion cores, but I'd certainly feel like I'd got my money's worth. Modder and YouTuber Cosmic Contrarian has already made several videos featuring thousands of Fallout 4 NPCs going to war, but the scale of this one is off the charts crazy. 20,000 extremely hostile Assaultron robots march on the Commonwealth, and a second apocalypse very much seems on the cards.
There's a plot of sorts, but really, we're just here for the carnage. There's lots of careful editing and I'm sure this would make the average PC beg for the sweet release of electromagnetic death (even Cosmic Contrarian's machine clearly struggles in places), so this isn't a scenario you should realistically expect to be experience first-hand, but it's a hoot to see what a Fallout 4 pushed to extremes looks like.
For once, I'm not lamenting that Fallout 4 is almost entirely a shooting game.
Cosmic Contrarian's other videos include the below 30,000 glowing Deathclaws laying waste to 300 super-mutants, which looks a bit Orks vs Tyranids from a dream-scenario Epic-scale Warhammer 40K RTS, and 30,000 standard Deathclaws versus the Brotherhood of Steel, which is Space Marines vs Genestealers if you squint a bit. There are more still on his channel, though they're not quite as spectacular as these.