That's for you to decide, on a number of levels. If you're not using it to store Pokémon you couldn't otherwise store in-game, and you don't want to pay another $5 for whatever reason, then don't re-up. Keep in mind, of course, that if you don't re-up, you'll need to remove your Pokémon from Bank - after a grace period, they'll disappear forever otherwise.
However, Bank is almost certainly not going to simply "become worthless" with the franchise's eventual jump to the Switch, which could itself still be the better part of two years away. Bank was designed as a cloud-based service specifically so that it would be future-proof and compatible with any future Nintendo hardware that can connect to the Internet, which one presumes will be all of them. The question of whether incumbent hardware can connect directly to new hardware to transfer Pokémon across a generational gap is no longer a concern.