In the x86 space, Meltdown absolutely was down to one company apparently deciding to over-optimize for performance.
I can't find it now, but I remember reading a thread from (I think) the OpenBSD devs about how the Intel MMU documentation described fairly sane behaviours and how far the reality deviated from the documentation.
Serious weasel wording, outside of x86 space every other high end architecture also had Meltdown issues, ARM, and IBM's POWER and mainframe designs.
ARM is actually a good target with a number of their newest designs using out-of-order speculative execution with Spectre vulnerabilities and their owning mobile space outside of notebooks, one of the newest even being vulnerable to Meltdown, but the significant headline worthy instances tend to come in much more locked down devices. Speed is also an issue, everything else being equal, the faster the chip, the faster data exfiltration proofs of concept will work.