jimmysmitty :
The biggest loss is that we never went to a pure 64bit uArch and are still tied down to the aging x86 design. While its nice to have for migration it doesn't help us move to a pure 64bit world. Many programs are still written in 32bit which don't take advantage of the biggest thing 64bit gave us, more than 4GB of total system memory.
Backward compatibility has it's cost, Even today, you can't install Windows 98 for example on a modern system, hell some system can't even start Windows XP or 2000, this is mainly duo to the move to UEFI from BIOS but even the CPU's now are different. Apple is leading the move in this regard, they stopped accepting 32bit iOS apps a while ago, Google followed them, even Apple now doesn't accept 32bit macOS apps. They're forcing the developers to move away from 32bit.
Both Intel and AMD slowly deprecate older instructions that no longer are required, then eventually will remove them all together, this happened before with 16bit and 32bit, starting from the OS (Windows) then moving to the CPU's.
Any company can make a big move like this, pure 64bit CPU. But moving the whole world to it is very hard and costly, nearly impossible.
Itanium while it was good, but it's performance wasn't that much to justify the move (cost, time and loss of backward compatibility), it's dependence on VLIW also made things harder compared to x86, you know x86 isn't just well known to developers, it's also easier to develop and optimise specially for the kind of software we're using.
Itanium x86/32bit code was basically emulation, suffered huge performance drop also to the point it was slower than most much much less expensive x86 systems, while Itanium was first meant for enterprise market, the later issue meant it will never find it self go to the mainstream.
AMD64 paved the way not just for the mainstream market, but it also promoted many enterprise/server market to move away from SPARC/POWER/Itanium to lower cost and easy to develop x86 systems. Intel later adopted the x86-64 with it's own implementation (they called it Intel64) and used it, the x86 market even in specialised market exploded then.