God of War Ascension is not a good example of multiplayer "ruining" a franchise unless there are two distinct games/campaigns which from a development standpoint means a division and typically a dilution of resources. One that can only be played by one player and the second that can only be played with more than one.
That's the basic model for the FPS genre, and to a larger extent, shooters such as Uncharted or Gears of War. In the instance of CoD, it was pretty clear that the single player campaigns were reduced to allow more resources to go into multiplayer, which in all fairness, is where the money for those games lies. Why? Longevity. Single player campaigns can be finished in a weekend. That's not worth $60. Most developers want players playing their games for as long as possible; few can rely upon the tried and true RPG design techniques that can stretch a game to great lengths.
RE5 was not an example of the single player mode being drawn down as it was a partner optional campaign plus an online multiplayer only mode. Sort of the best of both worlds, with the multiplayer mode being a bit more of an afterthought.
In the case of Uncharted, if anything, it just feels like more was added rather than diluted when looking at the jump from Uncharted to Uncharted 2. Better single player campaign with an all new multiplayer game. About the same between U2 and U3. Nothing taken away by any means.
I don't think Ascension will have any problems with resource dilution as it looks like it's taking the same multiplayer format that has existed since the old days of multiplayer arcade games. You can play through by yourself, and other players can drop in or out at any given time presumably with the difficulty curve adjusting depending upon the number of players present. How anyone could interpret that as "ruining" the franchise is a bit curious to me.