Sequels themselves aren't the problem. The problem is that they sometimes try an idea that doesn't work well with the franchise, they run out of ideas and just throw in filler, or something similar.

I think that for some games the sequals are good ideas. The problem is the storyline and plot for those bad ones.

I think it depends on how the sequels are made, metroid for example changed tremendously over the years and will probably continue to evolve, will it grow stale, probably. Maybe don't need any limits because a game under the same franchise can still be a completly new experience.

Alot of the time games dont even need sequals, sure if the ending is unresolved then maybe it deserves one, but there are sequals that are like "10 years later, even though this incedent was resolved, lets have a new adventure with that person".
It really is lazy, I mean, we're done with this person, what's wrong with "and he lived happily ever after"?
Sometimes the sequals just make you go "what was the point, everything was resolved, now this character that was cool just seems lame".

Dragonessa : says "Now whoever said FF7 is a flop must be high on something....though I do respect you, whoever you are....FF7 was the crown jewel of the FF series for like....ever...until FFX came around and blew it sky high but anyway....FF7 was the game that gave the rpg genre a much needed phoenix down to revive it"
Well I might have said similar in another thread, not in here I dont think.
But honestly it seems more like you are the high one , FF7 was mediocre, to say that it was the "crown jewel of the FF series" until X came out is ludicrous and ignores 2 games that are many times better then 7, FF8 and FF9.
FF6 was released in '94, FF7 was released in '97, that leaves only 2 years, '95 and '96, as any gap.
That's hardly enough time to say FF7 "revived" the genre, considering two huge RPGs were released in said "gap", Chrono Trigger was released in '95 and Diablo was released in '96.
And that's ignoring other RPGs released in that timeframe.

Sequals like Final Fantasy are ok, mainly because they are sequals in name only not in storyline or anything, though it might be nice if more of them had different names other then Final Fantasy and a number.
Maybe something like "game name" followed underneth with small letters "a final fantasy tale" would work.