Right, x86-64 offers eight extra register names† (r8 through r15). If you choose to go from x86-without-frame-pointer to x86-64-with-frame-pointer you gained 7 register names which is huge.
This makes the case where that one extra register name makes all the difference much rarer, arguably turning it from "I demand a compiler flag" to "Let's just hand-write the machine code for this one very special routine if our performance data suggests it's worth it".
† Internally a modern CPU has far more actual register, to enable a feature called "register renaming". But we can only talk about them using their canonical names, and x86-64 adds eight more of those.
This makes the case where that one extra register name makes all the difference much rarer, arguably turning it from "I demand a compiler flag" to "Let's just hand-write the machine code for this one very special routine if our performance data suggests it's worth it".
† Internally a modern CPU has far more actual register, to enable a feature called "register renaming". But we can only talk about them using their canonical names, and x86-64 adds eight more of those.