I would be surprised because even though Portnoy has been the one handling most of the fanbase and doing the bootlegs, interviews and such, he's still one member in a band.
Seeing especially as Dream Theater is a progressive band, and looking at the albums they've made, i don't consider them to be a one-mans band.
The songwriting is spread out, sure Portnoy wrote songs, but so did Petrucci, and the others (but in a lesser extent).
If it would've been an extreme case of the leader-figure leaving i would understand, but it isn't like Steven Wilson left Porcupine Tree, or Mikael Åkerfeldt left Opeth, or Daniel Gildenlöw left Pain of Salvation (just to name a few examples), DT will still sound DT because they still have 80% of the band left.
Even though they will probably try a few new things with a new drummer.
I find it hard to believe that there's fans saying like this: "i won't listen to DT anymore, because Mike Portnoy left", because it doesn't make much sense.
Sure he was more then 'just' a member, but i can survive without "DAY AFTER DAY"-parts in DT-songs.