Many of us stuck it out with T-Mobile plans despite better options at the time because we're okay paying more now in return for a guarantee of not paying more later.
If T-Mobile is not honoring this once it no longer benefits them, a minimum penalty should be the price difference between T-Mobile and the cheapest similar option over the lifetime of the plan.
If I paid $50/month, and there was a $30/month option through Mint, MetroPCS, etc. for four years with the same data / talk limit, I should get back a minimum of 48 months * $20 ≈ $1000 (increased by a bit, assuming money was held in an index fund in the meantime).
Part of the reason this matters is many companies use similar lifetime tactics to get started. Nebula has $300 lifetime plans, which provided much of the early capital they needed. If this doesn't work anymore, it pisses in the pool for everyone wanting to do something similar.
It's perfectly fine to have early customers fund you with (eventually money-losing) lifetime plans, to buy marketshare this way, etc. There are a lot of business models which go away if this can't be relied on.