Indeed,

but you wouldn't be thinking about instance sizes,

how many processes per instance and

wondering if you're hitting kernel limits with all the issues coming up

You'd probably be worrying more about instance sizes if you ran a single executor per container; the memory overhead of your app would become a problem very quickly unless it's startup footprint was quite small.

That's what they're doing now.

One app pool with one worker x number of cores

I assumed they're managing all those workers under one parent process which compiled their codebase on start. Perhaps that assumption was in error.

Why not?