I worked on a browser team when Spectre/Meltdown came out, and I can tell you that a big reason why Firefox and Chrome do such severe process isolation is exactly because these speculative attacks are almost impossible to entirely prevent. There were a number of other mitigations including hardening code emitted from C++ compilers and JS JITs, as well as attempts to limit high precision timers, but the browser vendors largely agreed that the only strong defense was complete process isolation.
I'm not surprised to see this come back to bite them if after like 7 years Apple still hasn't adopted the only strong defense.