Besides which, the Democrats are a coalition. Even if the GOP collapsed, it would be very difficult for the Dems to push any one agenda, as their support base is widely divided on almost all issues.
That would probably be the ideal result, but partisanship is quite high these days, and I can't help but think of examples where the modestly Democratic district votes straight party Democrat even when the candidate is clearly a gigantic crook/sellout/etc.
Actually, that's a huge problem in states overwhelmingly controlled by one party or the other. Take Texas, for instance. It's pretty well owned by the Republicans right now, and voters almost always vote on party lines, not ideology. So what keeps happening there is that the Democratic districts vote in Democrats and then a lot of the safely Republican districts vote in oddly moderate Republicans solely because they're incumbents running as Republicans. The opposite is the case in states like, say, Oregon or even Vermont.
Inversely, sometimes the "swing" states have far more ideologically pure candidates despite nominally being moderate. New Hampshire in particular has a legislature filled with alternating right wing and left wing radicals depending on election season. Incumbents are regularly kicked out, and neither party is capable of completely pushing its own agenda.