90s x86 from ISA pov is already free to use, no? The original patents must have expired and there's no copyright protection of ISAs. The thing keeping the symbiotic cross-licensed duopoly going is mutating the ISA all the time so they can mix in more recently patented stuff.
Sure, but why does the developer environment have to be the same architecture as in production? Think of it as ahead-of-time binary translation if you want to.
These days, even fairly low-level system software is surprisingly portable. Entire GNU/Linux distributions are developed this way, for the majority of architectures they support.
90% of those problems effect people like you and I, developers and power users, not "regular" users of machines who are mostly mobile device and occasional laptop/desktop application users.
I suspect we'll see somebody -- a phone manufacturer or similar device -- make a major transition to RISC-V from ARM etc in the next 10 years that we won't even notice.
I don't think it works that way in practice.
Some distributions like Debian or Fedora will make newer features (such as AVX/VEX) mandatory only after the patents expire, if ever. So a new entrant could implement the original x86-64 ISA (maybe with some obvious extensions like 128-bit atomics) in that time frame and preempt the patent-based lockout due to ISA evolution. If there was a viable AMD/Intel alternative that only implements the baseline ISA, those distributions would never switch away from it.
It's just not easy to build high-performance CPUs, regardless of ISA.
AFAIK, most of event x86_64 patents are largely expired, or will be within the next 6 years. That said, efforts for a more open platform are probably more likely to be centered around risc or another arm alternative than x86... While I could see a standardization of x86 compatible shortcuts for use with emulation platforms on arm/risc processors. Transmeta was an idea too far ahead of its time.