I wouldn't say that. ARM as splintered as the x86 world, if not more so, with processors implementing some combination of softfp/VFP/NEON/Thumb/Thumb2/ThumbEE/J

I wouldn't say that. ARM as splintered as the x86 world, if not more so, with processors implementing some combination of softfp/VFP/NEON/Thumb/Thumb2/ThumbEE/Jazelle, not to mention multiple ABIs. Yes the x86 camp may have different feature sets but it will be handled like all previous extensions to the architecture: these features will be expected to exist (for single deployment targets), detected via CPUID and code that uses these instructions will be selected appropriately at runtime, or they will be ignored.