Yep. It's going to be 5.5 unless it's completely incompatible with 5e. 6e will be incompatible.
4e had the absurd notion that every book was core.
Are you talking about the erratas? If so, and they changed the PHB, those were changes to the core of the game and altered the game, however small, from being what it was. Those were fundamental changes. 5.5 will be significantly greater.
There's nothing arbitrary about a reasoned definition.
As lomg as you enumerate them with .x then I agree.
I think "fundamental" is too big of a word to describe minor changes... but yes, they are changing the core or fundament, as you call it.
I would not call it fundament, as I don't see every little rule of the core book as fundament, as I don't see every stone in a house as fundament. Only those below the surface that are necessary to keep the structure functional.
So I would make a difference between:
Core book changes,
structural changes,
and fundamental changes.
I would see core book changes in the form of little errata or update to a class or subclass as not warranting any change to the edition name.
I would see a structural change as a reason to increase the .x.
I think a fundamental change will increase the number before the floating point.
3.5 is barely below that mark, hence the jump to 3.5. There were massive changes in the structural layer. There were a few fundamental changes in the mentality, but only going from the rules, characters were quite compatible.
4essentials was only barely a structural change. Every class of core 4e could still be used.
OneDnD falls between those two, going from the playtests we have seen.
A few core rule changes, but every character can easily be played with the new rules.
If you want to grapple, just don't use athletics, use unarmed attack instead. Exhaustion? Just use this effect.
Most character sheets donvt have to be altered, as grapple is not noted explicitely anywhere. Neither is exhaustion.