I am a long-time Fastmail user who also was on the "lifetime" 16MB plan. Last year I switched because I wanted some features that were only available with a newer plan. I was a little annoyed to have to give up my lifetime plan but in my case it was more understandable.

I've been very happy with Fastmail for over a decade and this seems like one of the few missteps they've made. New "lifetime" 16MB accounts have not been available in years. It really doesn't seem that there could be very many of them still in active use that it would hurt anything to just continue to grandfather them, especially since they were sold explicitly as "lifetime" one-payment accounts.