[Master Belisarius]

I hate the 100 turns limit like everybody here... but guys, business are business. And in my humble opinion, I think that MM can lose more clients without the 100 turn limit, rather than keeping the limit.
A demo it's only a demo, and at least this is a playable demo.
I can remember that the Alpha Centaury (SMAC) demo had a 100 turns limit, and SMAC sold a lot of copies, although was really a very limited time (it's not possible to finish a SMAC game in 100 turns, but I have finished a LOT of SE4 games before the turn 100).
If you start games with 5 or 10 planets in a small galaxy, and all the tech, it's pretty easy finish a SE4 game... Sure, if you want to play epic games in large galaxies, starting with only one planet and without starting tech, then you will be unable to finish the game... but as I said, it's only a demo.
By the way: in my multiplayer SE3 games, I never have played a game during 100 turns... the more large game was finished in 80 turns.