Regarding the audio engine, it was rewritten for Cubase 8, that would be 5 years ago. It’s stated that the entire engine was completely rebuilt.
Statement from Steinberg, at “What’s new in Cubase 8”:
Massive engine rebuild: Cubase 8 is probably the most efficient DAW software available today. For this version we > completely rebuilt the entire engine > that runs Cubase to make it much faster, more fluid, more snappy.
Anyway, “gapless” didn’t make it in that rewrite, obviously. But I doubt that really another complete rewrite is needed to make it “gapless”, as speculated about here. It should be less drastic (but I don’t know of course).
And the Pool window. Yes, lots of windows (and lots of plugins) have been updated for the last few versions. A few left to handle to be done.