Sort of. In the replatforming/codebase conversion case, the amount of care and labor needed to effectively use shared memory for communication is, in my experience, much less than the care and labor needed to make things work in the same address space (and the labor and cost-in-currency of making things work with sufficient performance using traditional IPC is also often preventative). In that regard, I don't think it's equivalent to arguing in favor of C: capable alternatives to C exist that require less care and labor to use; capable alternatives to shared memory IPC in some cases do not.
I'd be curious how much effort and specific shared-memory-IPC-expertise is required on the part of the Postgres maintainers to keep the shared memory layer capable and secure. I hope it's more like iceoryx2 bills itself in that it's relatively well encapsulated such that folks can use it safely without extensive familiarity with the risk domain, but I might be disappointed.
Regardless, I hope we can agree to disagree on specifics; I appreciate the thoughtful response in any case.