I guess backwards compatibility isn't important to a certain level of consumer, but it might matter for professional / business applications?
It has less importance in embedded applications, which not coincidentally is where the various failed processors which dropped compatibility live on.
Arguably things have changed with managed applications and walled gardens like the iPhone for consumer use, which eventually leads back to a discussion about whether personal computers will survive at all. The various companies selling walled platforms sure say they will not, but the RISC vendors said the same about Intel's x86 and Microsoft Windows, and look where they ended up.
ARM is now doing to Intel what Intel did to the RISC vendors with economy of scale pushing performance up from the low end, but I am not convinced that will be enough to displace x86 if ARM systems remain closed, or perhaps "curated". And any advantage from a simpler ISA and simplicity from lack of support for backwards compatibility may not be enough. That is a fancy way of asking, "where is my open ARM desktop replacement?" And like the failed RISC vendors of the past, companies that do not make desktops, including Apple, say that I do not need one. Well of course I do not need one if they are not making them; just ask them!
Apple has changed the CPU family in "Macintosh" computers three times now i.e. used four different ISAs: Motorola 68000, IBM PowerPC, Intel x86_64, and now ARM Aarch64. Actually, five, as there were a couple of 32 bit Core models at the start of the Intel era.
Each time they have for some years provided software emulators for the old ISA which have been pretty transparent.
For example, it was possible to run a PowerPC version of MacOS 9 on an Intel Mac up until Snow Leopard was replaced by Lion in July 2011 -- and you could run 68000 apps in that MacOS 9 using it's own built in emulator.
I think that explains why Apple is now primarily a maker of phones and consumer electronics, who just happens to also make some personal computers.
I wouldn't really expect it to happen any time soon. It is still a lot of work to replace something that isn't broken, but I don't think the performance or compatibility would be nearly the problems people had trying to do this 2 decades ago.
The various processor manufacturers crushed by Intel's lower performance x86 always said, "just recompile!" Of course you could run JAVA on anything, right? RIGHT?