Is scsynth powerful enough to run dozens of channels of audio?
Could a traditional DAW (like Ableton or FL Studio or at least LMMS) be built using scsynth as the sound engine?
To answer your first question, yes. The resource cost of an unladen Bus is negligible. The main practical limitations lie with running too many CPU-hungry ugens at once. You can run 50 Buses easily, but not 50 reverbs.
If you're talking about large numbers of audio inputs or outputs, SuperCollider can handle that easily. If you want to deal with spatialization of microphone or speaker arrays, I recommend the Ambisonic Toolkit.
A full-fledged DAW is a pretty big project (as it would be in any other programming environment) but it is possible, and has been done! This year, Neil Cosgrove released LNX Studio, a GPL-licensed DAW written entirely in sclang. Right now it is OS X only, but Linux compatibility is underway.