Why would programs want to "depend on the invariant that the size of a cache line will stay constant"? That's a minor detail that most applications don't care about.

In this case, it probably mattered because when JITing code, you need to explicitly flush instruction cache after changing code. But that's not anything the application cares about; that's something the runtime cares about, and the runtime is going to want to support taking advantage of the speed/power tradeoff without making application developers have to go to the trouble of thinking about it.

Also, from a user perspective, you don't want applications to be able to demand only the big, powerful cores. That will eat through your battery much faster, with likely very little benefit.