As someone who wasn't a fan during BC&SL, but as someone who is a fan of a band who went through a similiar situation (launching predictable albums in a row, before a bandmember left and everything changed), I can say that I would have felt like it's just "Oh, it's DT, another album", and so on, and not the same level of hype I had with TA.
If Portnoy hadn't left DT, the band would have probably written an album with no epic, one of the reasons for splitting ITPOE in two parts was because in Octavarium they already had a closing 20+ epic, so naturally they wouldn't want to write another epic. And remember, not only did they do this twice, but THREE times, as TCOT is basically another epic (but since 4 songs in that album are 10+, it doesn't feel that epic).
They always tried new things, concept album, double album (albeit not intentionally), metal album, thematic album, and SC was the first one where they arrived clear minded. BC&SL I see it as a return to themes, as I feel it as a very homogeneous record, with the nonfictional personal topics and overall style.
I really can't think what they would have tried next, probably a more power symphonic metal, I have always wondered how would it be if DT went Rhapsody or Kamelot/Angra for a brief moment, with their own touch of course.