That's not the troubling case.

The troubling case is the company hosting the DB shutting down, and therefore the entire system becoming worthless. This is what prevents you as the customer from trusting such a system.

That is preventing you to trust a system where "a gaming streamer is rewarding his most loyal viewers with tokens they store in their own database and you can redeem for shoutouts or merchandise."?

I would somehow think that the database technology is quite secondary to the creditworthiness of the gaming streamer when people decide whether to trust or not a token system of a gaming streamer.

It's the streamer who's trust you need. Whether they have their patron's trust is a separate issue.

but this is what exports are for. I would be fine if gmail etc. shut down tomorrow. there's no reason to add a massive overhead to everything just to solve an already-solved problem.

What if it’s not a massive overhead? What if the tooling is as simple as setting up a database nowadays, and the performance is a known/acceptable trade off?

Surely you can see there are some advantages of blockchain based tech then?

It's just a bit more effort. All companies sell convenience.