I would not think to negatively about this. If the price was right, you would probably get a serious boost in capital in a short amount of time. Let's just take a number and say it would cost about 5 licenses worth (upgrades). That would give you 100$ plus (?) for each person that decides to be faithful to your MC. For new customers it would mean 1 full purchase, and about 2 years of upgrades worth?
You will start "loosing" some of your normal income after a period of 3-5 years, because of the long and medium time users no longer have to pay. The great thing about this though, is that if a noticeable portion of your customers goes for the lifetime sub, you'll have an opportunity to spend much of that extra cash on staff and other expenses. And that can give you a great boost in productivity and feature development. Making the application more popular and better know in the market, at a faster pace than ever before. So, counting the potential new customer of an improved app, up against the possible "loss" of upgrade sales of some of the returning customers, it might be very beneficial in the long run. At least that is what I think.
I'm by no means an marketing or sales expert, so it might not be as easy as it sounds. There might be potential traps with such a purchase alternative, but I just wanted to say that it might be worth considering