I didn’t put this in the rules section here, because PMC is not a “rule”. It’s a guideline for aesthetics.
PMC says it is something to aim for to maintain the aesthetic of a mass-battle / R&F wargame. It also says that if it’s not practical to do so, then that’s fine. Nobody is expecting anyone to rebase existing armies and if scenery or large models or anything else means that 75% (or more, up to FMC) is not practical, that’s absolutely fine.
The only time this is likely to be an “issue” for anyone is when they want to win best painted army or a trip abroad at an official Mantic event and it’s been stipulated that PMC is in effect. Even then, the “not practical” thing will be taken into account for model size or scenery restrictions because of a theme in the army.
TOs of other events will obviously continue to allow whatever they like.
So there’s no need to panic