They could raise the price or start having ads even if you pay. You're a renter. You have no rights.
Not sure why you need to mention the "renter" thing, this is literally a service you're paying for and that you can stop paying the minute it stops being useful to you. There's no vendor lock-in, just stop paying for Premium when if it ever becomes shit.
The OP is not saying Premium will always be good, they're saying why they find it useful to pay for Premium now.