I think I'm in the minority here when I say I didn't much care for that feature. I didn't like
Pikachu following me around in Yellow, and I didn't like it even more when I couldn't do anything about it in HGSS.
HOWEVER, this is not why I don't think it's likely to comeback.
These days, the expectation is for the graphics and world to take full advantage of the hardware capabilities the platform offers. Especially for AAA titles like Pokemon. Now lets consider what goes into the follower mechanic.
- The follower must not be overly detrimental to movement, and cannot create problematic save states
- The follower must represent a recognizable version of a given pokemon in the party
- The follower must be able to access the same pathing as the player
The first is foremost to prevent the player from becoming stuck or clipped out by their follower, obviously. But there was some experimentation in the speedrunning community with the follower mechanics when HGSS came out, which was met with varying degrees of success. Nintendo will probably want the newer, more mechanically nuanced followers to not present many, or if possible any game breaking exploits.
For the second, we can consider the options of sprites, or 3D models. Sprites would be kinda cool to a niche audience. But most people would probably want 3D models. It's possible that the battle models could be recycled (like during the trade sequence), but if they can't for whatever reason, thats over 800 models that need to be redone. Even if they can, think about all the man hours needed to give them all proper walk cycles at varying speeds. Imagine how it would break immersion if they didn't do this, or worse, did it poorly. That's a lot of money out of the budget that can go to other features or aspects of the MDA.
The last part pretty much kills the sprites possibility. Sprites would probably have to be gridlocked. Meaning they couldn't path the same way as the players. There are a ton of programming issues that could result from a lost follower, especially save glitches and wrong warps. The game has to store and keep track of every entity, and all data can be exploited (see: any speedrun with a wrong-warp).
I'm not trying to be a naysayer to something that a good number of people clearly want, but thinking about it from a design perspective, I can't help but think that this would probably be something I'd cut when push came to shove.