The bottom line is that without upgrading their networks, they can't provide the promised service to 100% of their customers.
Wrong (AFAICT).
They are actively throttling users. That is not the same as their network being unable to handle it, or for congestion to affect many users.
The users with metered plans are not being throttled. They may be using even more. Everyone could do that, and they would not throttle the metered users because they want that additional money. The unlimitted users are getting throttled when they hit some cap of MB/month. That's not unlimitted. Unlimitted would mean they should behave just like the metered plans, but they'd pay a flat fee.
As others have said, they should just terminate all of these contracts and offer those users something else. They are all on month to month. There's just an awful lot of them that ARE still profitable, and they're scared to lose that... so either it's worth it to keep them all or not, but they shouldn't be throttled like that (as much as I hate the idea of some very small percentage of folks ruining my day to day experience).
Personally, I'd like a more customizable rate... something like the way fractional T1's used to work (dedicated 256k up/down, and burstable to 1.5mbit if it's available... but some different rates in those places, especially on the high end). I'd be willing to wager this is quite possible (for 3g/4g/lte as well as cable/dsl/etc), but is just "too complicated" to market to people (quantity, 2gb/month, is easier to grasp than throughput, 256kbps; and they are very very different forms of measurement, with the former barely meaning anything - if you do all your downloads at 3am, you'll still hit your cap even though there was loads of extra bandwidth).