it will probably be a similar situation to x86, with various vendors implementing various instructions in some processors that won't be supported by all. I guess the difference is that there may be many more variants than in x86, but performance-critical code can always use runtime dispatch mechanisms to adapt.
It's true that there are extensions to x86, but 99,99% of software out there (the one you'd commonly install on Windows or find in Linux distribution repos) doesn't use those instructions or maybe just detects the features and then uses it.
I don't recall encountering a "Intel-locked" or "AMD-locked" application in more than 20 years of using x86. Ok, maybe ICC, but that one kind of makes sense :-)
That doesn't mean compilers will emit such instructions; maybe hand written assembler will become less portable if such code is making use of extensions...but that should be obvious to the authors...and probably they should have a fallback path.
