I have an S4 with a mostly dead battery. Can't be bothered to take it in for repairs, so I'm using my S3 Cellular until I can buy an S7 later this year.
Luckily the cellular model has more storage so I haven't dealt with this issue, but I agree that this model shouldn't be sold. It's far too slow and overall I use it (and like it) a lot less than my S4. I don't have any music or apps synced to it at all. I basically use it to check notifications, control music, and use Apple Pay.
Generally Apple tends to update products for a couple of years after they were last sold, but can anyone imagine it getting Watch OS 8 or 9?