There are several things that the series has actually done WRONG (as opposed to merely keeping it the same) as it moved to later titles. One of the biggest mistakes was returning to 8 badges per game. The reason that worked in the original was because it was THE original game, so absolutely everything was fresh. After that freshness wore out, they should have made the standard 16 badges so that a long and engaging single player would make up for the loss of its freshness.
Another mistake was moving away from sandbox gameplay. Along with the 8 badges, the games have increased the sense of guidance in the single player, so that the player crosses more areas during the normal story but doesn't feel like he/she has as much opportunity to explore. One of the most genius innovations in the original was the Surf HM, because it added enormous value to backtracking in the form of new potential areas, new potential Pokemon, and simply an excellent addition to the sense of exploring and searching that the series was built on. Unfortunately, they never made a similar innovation again. Their later HM moves are in fact even more disappointing - Rock Smash is basically a copy of Cut from a gameplay perspective, and moves like Waterfall and Defog are entirely situational and tend not to be used much anyways. Dive was a great idea, but they never really used it in the one game it appeared in.
In short, the problem the series suffers from is that the team has focused far too much on the battle system and has allowed the rest of the game to either become stagnant or even regress in ways. The multiplayer metagame is better than ever, but the single player experience, which is a far greater appeal to the majority of the Pokemon fanbase, has suffered some.
I'm not saying there aren't people who don't enjoy the improved elements. What I am saying is that the improvements are alienating part of the fanbase to appeal to other parts, which is the opposite direction the rest of Nintendo is going. I guess that's just what happens when you have a franchise as unquestionably successful as Pokemon.
I sometimes wonder why the hell I write so much whenever the topic is about Pokemon. Probably because it receives way too little attention and recognition considering its importance to the industry....
