In any sane world I would be burnt out of this game 100x over, almost amazes myself that I'm still playing. I think it's the sheer amount of time I've put into it, makes it really hard to imagine stopping...I kind of wish Gungho would have released Puzzle and Dragons 2 by now, not sure what they could possibly be building that is taking 4 years to do.
The sheer # of dungeons in JP is overwhelming and many of them are difficult (/whine mode) - I know they do it so that the top tier does not get bored but still...I actually kind of enjoy the "collect them all" aspect so would prefer they focus more on collabs.
GH has gone on record (I think in the JP stream where multiplayer was first announced) stating that at one point that they were developing PAD2, but did not follow through for the exact reason you mentioned in your post. Too many people have too much time and money invested in PAD to make them throw it all away for PAD2. Releasing PAD2 would probably go poorly (see most F2P sequels bombing) and would kill PAD1 at the same time through cannibalization. So, PAD1 will continue into the foreseeable future.
At the same time, that is a curse. Games are becoming more graphically intensive, and PAD is an old game. On one hand, the graphics have improved in newer dungeons considerably. On the other hand, many long-time players are still playing on older devices, which makes the game unplayable (hence the Simple Background Option). Legacy concerns do have a material impact on the growth of the game in the future and hampers development. At the end of the day, GH will priortize paying customers over fancy graphics.
In JP, PAD is approaching 5 years old. So, yeah, there is a crazy number of dungeons out there. People expect new content constantly to keep them engaged in the game. To counter this, GH has pretty much fast-tracked new players through most of the old content through leaders with crazy leader skills. Further, post patch 9.1, new accounts can simply be carried through old content to speed them up even faster.
To that extent, most believe at this point that the situation is fairly transparent that PAD was never designed to have a lifetime of 5 years or more. As such, silly broken concepts are now happening that did not exist before. Cards can hit for 8-9 digits each now. Others can create on-demand 75% shields to tank practically anything. Some suggest that multiplayer was created to give GH some additional room to play with card growth before hitting the absolute wall.
No one really knows where the game is heading in the future, but it has definitely shifted towards collecting the "right" cards needed to build the optimal team rather than any particular playstyle. The June Bride dungeon is an excellent example of using collabs to deploy niche variants of popular cards that are more-optimally effective on very specific teams. Think of it this way. Pantheon gods cover the base foundations for many teams, and are always available through the REM. Collab cards offer specialist versions of these gods that are more valuable on very specific teams, and are much harder and rarer to pull than their standard pantheon equivalent (in most situations).
