Except that Sprint and T-Mobile do offer unlimited data plans (without throttling) and T-Mobile simply throttles instead of charging overages for its non-unlimited plans. I've used both and have yet to encounter issues with congestion (though I don't live in a big city, so maybe it's different in that environment).

Non-unlimited plans with throttling instead of overage charges would probably be the best solution to overcrowding issues in the short-term.