> there are plenty of software developers who offer lifetime purchases. In fact there was a time that subscriptions for software were virtually unheard of.

There's a big difference between software you buy, run on your computer, and don't expect to be constantly updated, vs. an online service that you expect to stay up and serve you new content forever. In the latter case, if a customer drops off after 24 months, that lowers your costs. You can't reasonably charge a user for the number of months they could potentially remain subscribed.