Technology

There are some memory latency benefits to putting memory on a single chip, but to date, that's largely been handled by adding cache memory to the CPU, and later adding multiple tiers of it, rather than eliminating discrete memory.

The first personal computer I used had 4kB of main memory.

My current desktop has a CPU with 1MB of L1 cache, 16MB of L2 cache, 128MB of L3 cache, and then the system as a whole has 128GB of discrete main memory.

Most of the time, the cache just does the right thing, and for software that is highly performance-sensitive, one might go use something like Valgrind's cachegrind or something like that to profile and optimize the critical bits of software to minimize cache misses.

I could believe that maybe, say, one could provide on-core memory that the OS could be more-aware of, say, let it have more control over the tiered storage. Maybe restructure the present system. But I'm more dubious that we'll say "there's no reason to have a tier of expandable, volatile storage off-CPU at all on desktops".

EDIT: That argument is mostly a technical one, but another, this one from a business standpoint. I expect PC builders have a pretty substantial business reason to not want to move to SoCs. Right now, PC builders can, to some degree, use price discrimination to convert consumer surplus to producer surplus. A consumer will typically pay disproportionately more for a computer with more memory, for example, when they purchase from a given vendor. If the system is instead sized at the CPU vendor, then the CPU vendor is going to do the same thing, probably more effectively, as there's less competition in the CPU market, and it'll be the PC builder seeing money head over to the CPU vendor

they'll pay a premium for high-end SoCs.

In Apple's case, that's not a factor, because Apple has vertically-integrated production. They make their own CPUs. Apple's PC builder guys aren't concerned about Apple's CPU guys extracting money from them. But Dell or HP or suchlike don't manufacture their own CPUs, and thus have a business incentive to maintain a modular system. Unless one thinks that the PC market as a whole is going to transition to a small number of vertically-integrated businesses that look like Apple, I guess, where you have one or two giant PC makers who basically own their supply chain, but I haven't heard about anything like that happening.