Really? Of all your complaints that one seems the least true, in my experience. I think plots are lovely by default, but then 5% of the time it’s crap and then I’m cursing, with you, at how alien and annoying the whole thing has become.
I used Mathematica in 1988 on a Mac and have been grateful that my employment has given me free access ever since. The notebook interface feels basically the same to me as it did in 1988, when it was an incredible innovation. Now, it is a little annoying.
I also don’t know what feels unfinished about it, but it sounds like you’ve spent more time wrestling with it than me. It has been an incredibly enduring product that still wins at symbolic math. And I also hope open source tools can replace it in my work.