3D would make it worse. Lots worse. As we have it now, the symbols give us an easy-to-read view of exactly what is where, and the organized grid of ASCII or tiles makes designing and building very straightforward. If you tried to make it look more realistic, you'd need lots more space for each tile. Your screen would cover maybe a 20x20 tile area, which is not nearly enough to see a large area of the fort at once; or, if it could cover more, the graphics would be too small to identify effectively, unlike letters, symbols, and simple icons, which are easy to identify even if they are small.
Then there's the problem of processing power. Dwarf Fortress already uses up a lot of processing power without graphics, and most games use up most of their processing power on graphics. In order to increase the graphics in Dwarf Fortress to even 2000s standards, you would have to simplify the game, making it much smaller. No more 100 z-levels on a four-tile embark; now you have twenty. Did you want two hundred dwarves? Too bad; your limit is now thirty. Sure, you could--and probably would--reprogram the game to run on newer machines, but there's only so much you can get out of that. Eventually, you have to start cutting content, removing possibilities. Temperature, weather, and cave-ins would probably be the first to go. Units would have to default to hit points instead of complex skin, muscle, bone, and tissue layers. Contaminants and syndromes would vanish, probably replaced by a generic sickness/poison system.
Yes, you could turn Dwarf Fortress into a high-graphics game; but that's already been done. It's called The Sims, and while it's fun, it doesn't have magma.
As for DF going mainstream--nothing wrong with that. The more the merrier. It's already fairly popular. But not at the expense of turning it into something much less unique and interesting.