Given the nature of their sound, to achieve true mainstream success, they'd have to do what Genesis did in the 80's, which was to shift more towards more catchy singles, with only a handful of truly progressive songs on each album. I don't see DT doing that.
However, I can easily see them fitting in the mode of Tool, continuing to write more prog oriented material, yet getting more attention from doing that. Actually, DT has had some radio friendly material from the past. If they had released a song like Wither in the early 90's, it might really have been a major hit. If their new album continues the path that they've been going, they'll continue to become a bigger metal band, perhaps even getting to headline bigger venues. But I certainly don't see them rising to the level of Iron Maiden, Judas Priest or especially Metallica.
To get to that level, they would have to get a killer tour that could attract new fans. The big problem would be the type of tour as I'm not certain there are too many bands at a higher level than DT that could really be compatible enough with them for such a tour to work. To a certain extent, I think touring with Maiden has helped (as that is probably the closest). Opening for Metallica could also break doors down, although Metallica fans had been extremely harsh to bands they didn't like (such as the Cult).