They have? I like a number of Japanese RPG's, but they haven't particularly been refining anything. My understanding is that tabletop games never caught on in Japan, so Japanese RPG development is extremely insular, with their only frame of reference for RPG's apparently being the first few Wizardry games that were imported in the early 80's and Korean MMO's.
Korean MMO's as reference? Only for mmorpg, right? I hardly see any similarities between your average Korean MMO and JRPG, besides grinding, but JRPG did that first(between those two, I mean).
Now, Wizardry and Ultima are their frame of reference. Well, the first JRPG started based on these two(Dragon Quest, for example, is Ultima meets Wizardry). Then many JRPG were based on Dragon Quest, FF for example. And then any JRPG released after was based on DQ and FF. So yeah, if you go to the root of all JRPG, it all started on Ultima and Wizardry. That's why it's not wrong to say that JRPG is all about Dungeon Crawler. What kills JRPGs for me sometime is not its anime graphics, it's how some battle mechanics are boring, taking long ass animations for battles(FFIX is my second favorite FF, but god do I hate waiting 3 minutes to start a battle in that game), and some take the fun out of you(Tales series taking control of your party, for example). As for story, the more you play JRPG, the less you expect something from them. JRPG has a thing for them, though: when they do a good game, it's a fucking great game, see Souls series, for example. Or SMT: Strange Journey, if you love Wizardry-clones.
If for every 50 JRPG, I got 1 great game, then that's ok. There are like 100 JRPG being released every year, anyway. Wish there was as many WRPG being released now as there are JRPG(even if mostly were shit, I'm all for quantity over quality if I have games to play). What is the point in waiting 5 WRPG per year if they are boring ass RTwP or Bioware-wannabe game? I'd kill to have Wizardry 6~8 clones being released every year, I wonder how much JRPG would have changed if Wizardry 6~8 or SSI's Gold Box had be a big thing there. Grid-based battle turn is so fucking rare in JRPG that is depressing.
Still, I can't point exactly the time that JRPG started becoming a teenagers sving the world thing(keep in mind that this love for teenagers in Japan is strong in its society, since being an adult in Japan means you are a workaholic that hardly have time for any fun, thus, being a teenage again is the same as having time to do things you love and enjoy, so there is a reason there for them doing it). But for some, this killed any chance of them enjoying JRPG while for others this is the reason they do play it. But at least, pantsu jrpg is a thing there.