[TheCapeGreek]

>They all need stats

>They all need a moveset evolution

Neither of these have to shift a substantial amount between generations. The current stats & moves structure solidified by gen 4 already.

>They all need a habitat

No they don't. Trade them in and out of Home/Bank/whatever to move across generations, trade with other players, etc. This was standard flow up until the latest few generations.

There is not much extra game design consideration needed until you're talking about new features. But a good percentage of mons are still in the new games, so is it really that much extra effort to balance when you have new features in play? It can be as simple as just not giving older mons the same new abilities. Not every one got mega evolutions, for example.

Gamefreak are just lazy. Employee reviews seem to confirm this: https://www.youtube.com/watch?v=hFWjhfhJJqE