You're right it is very personal. We all tempt to like the things as we are used to as that feels more natural, but of course many other people use it and have different opinions so whatever we choose someone won't be happy; we can argue to make the "someone" a small group, but even so that choice might not be the same as you would pick in a "truly better" strategy. I firmly believe the only strategy which guarantees the least annoyance is the no-change: too bad we got it "wrong" (even assuming it is) initially.

For the sake of fun argumentation only: The think with splitting .impl only removes the "self documenting" aspect of it as you won't have an impl vs. spi but just an impl standalone in a long list of other API packages.. it's not standing out as special, we can't use a red font.