How useful are smart contracts if, by design, aren't supposed to be upgraded? This also excludes fixing a vulnerability

This question bugs me for a while. The only benefit from smart contracts I see at the moment is that you can check the rules of the contract as they are fully transparent. On the other, this transparency makes it easier for a potential attacker to misuse the contract increasing even more the need for upgradability.

What are you thoughts? Or is there a solution? The maintainer of the contract might deploy an updated contract and redirect request from the old to the new one. But then is pretty centralised and left to the maintainer

Edit: Thanks for your feedback. Proxy and diamond pattern are the keywords here. I'll have a look