T-Mobile has an unlimited everything plan for $80/month:
Cell Phone Plans | Cheap Cell Phone Plans Unlimited Data | T-Mobile
But you could just go ahead and use the $60/month with 3GB at 4G. That plan includes data rollover, so you're fine even during months when you use more data.
That saves you $720 per year, or $1,440 every two years. That's plenty of money to buy new phones with.
Look at T-Mobile's family plans. 5 lines with unlimited everything is $140/month. Over 2 years, that saves you $2,160. (edit: oh, that was with unlimited data on only one of the accounts. But presumably not everyone uses more than 5GB per month...)
I'm not sure what the big deal with grandfathered unlimited plans is. Most people who have it, don't seem to use anywhere near 5GB even. And holding on to it because it might become more valuable in the future is crazy: the carrier can stop offering it at any time, and will do so when it actually does become worthwhile.