In general, no they aren't required. However, if we're talking 5% of your customer base, you might not want to lose them.
In Boost's case, they might legally have to. Dish bought Boost as part of T-Mobile's deal to buy Sprint. I don't remember all the specifics of the agreement or what was committed to in terms of non-VoLTE Boost customers. T-Mobile had to give Dish 6 months notice before shutting down the CDMA network and they gave 14-months (now 17 months with the 3-month delay). I don't know if Dish committed to upgrading those customers as part of the deal (I know that it's their responsibility, but I don't know if they're required to upgrade them or if they can just wave goodbye to customers that don't want to migrate).
Ultimately, I think Dish is at least informally required. Dish is likely going to be looking for goodwill from regulators in the coming years. They have major build-out requirements coming due soon and they don't seem to have made much progress - and they have licenses worth tens of billions at stake (worth more than the company's market cap). Angering regulators at this point isn't a great move for Dish.
