"x86 bitcode" can be transpiled into "ARM bitcode."
Yes, you could do the same thing with a plain x86-ISA binary, but such a binary might have sections that are very hard to transpile efficiently/effectively—because e.g. they exist as the result of compiling hand-optimized x86 ASM source modules. Targeting bitcode constrains the inputs a bit more, such that the input to the transpiler won't be using extremely-microarchitecture-specific instructions.
And yes, the results wouldn't be 100% efficient. But it would be efficient enough—like Rosetta—to serve as an effective stop-gap to allow ecosystem consumers to continue to consume their purchased apps on new devices, until ecosystem developers upload explicitly re-optimized apps. (And it would—like Rosetta—at least let apps from "dead" development studios run, rather than killing them off entirely.)
Yes, you could do the same thing with a plain x86-ISA binary, but such a binary might have sections that are very hard to transpile efficiently/effectively—because e.g. they exist as the result of compiling hand-optimized x86 ASM source modules. Targeting bitcode constrains the inputs a bit more, such that the input to the transpiler won't be using extremely-microarchitecture-specific instructions.
And yes, the results wouldn't be 100% efficient. But it would be efficient enough—like Rosetta—to serve as an effective stop-gap to allow ecosystem consumers to continue to consume their purchased apps on new devices, until ecosystem developers upload explicitly re-optimized apps. (And it would—like Rosetta—at least let apps from "dead" development studios run, rather than killing them off entirely.)