- KSM only works on pages that are identical; I am skeptical that it can actually identify 2 instances of a lib after it's been through an optimizing compiler+linker (mostly LTO)
- KSM has performance overhead
- zram/zswap only let you reduce the impact of running out of memory at the cost of performance; you're still swapping, no matter how much we improve the details, so it'll always be worse than outright deduplicating libraries
I disagree that swap is only (or even primarily) for situations where you're running out of memory, though. It is useful for reducing I/O contention and improving memory utilization by replacing underutilized pages with file cache when it makes sense to do so. See this for more details: https://chrisdown.name/2018/01/02/in-defence-of-swap.html
And yes I sort of agree with swapping unused stuff so you have more room for cache, but that's only good for unused stuff; if you swap the programs you're actually running - even just zram swap - you're going to hurt performance.
Zram/zswap isnt nearly as good as the kernel knowing straight from the outset that all processes want exactly the same mappings from the same files.
I think that having a sophisticated desktop OS where every process had distinct physical memory for duplicated uses of the same code would be problematic at scale, especially on systems with less RAM. At least that physical memory would still be disk-backed, but that only goes so far.
That said, the security argument is also a good argument!