For reference Red Hat charges around $425/yr for extended support. Obviously the situation is different because Red Hat has a lot more software to support but they also have a fewer products and more customers that care enough to buy it. But I think the upshot is that a $5/yr extended support contract is a bit of a pipedream.

Just as an order of magnitude estimate we're assuming that it's about 100 times more effort to continue maintenance of a general purpose Linux based OS than a general purpose Linux based OS on specific set of hardware. And that's before you realize that the market for this support contract is just frugal IT people with specifc Android phones that haven't worn out from use. So not much luck with a 'we'll make it up in volume' analysis.

Google is maintaining Android so why would you assume it would cost massive amount of resources for Samsung to port or backport some security fixes now and then? The comparaison with Debian does not make sense.

Exactly. Why did you think Oracle wanted to muscle in on that?

There is Lineage OS which supports many Samsung devices with the latest android, completely free of charge.

Maybe those little phone repair shops should offer installing it for a small fee as a service.

There is also postmarketOS which attempts to bring a standard Linux distribution to old phones, but it's still in an early stage.

LineageOS? you have to download its roms on shady websites. No thank you.