That would increase the cost of the phone, since they would have to support and test the phone long past the expected lifetime. But, this is marketed as a budget phone, so they aren't going to do that.

That’s why he said paid for.

This is a budget phone, so there are very few people who would purchase extended security support after three years. There certainly won't be enough to cover the business costs for keeping the support and testing of an old crufty phone used by few people.

It's unsustainable to put the costs on the backend, so phone's price would have to be raised to cover those extended security updates. But, that won't happen for a budget phone. There just won't be enough people willing to pay after three years.

That's the problem with purchasing a budget phone. You get what you pay for, a phone that will go away soon.

Unless the price of the upgrades cover the support. Yes, of course, it would be a gamble, because nobody can predict what the demand would be in 3-4-5 years. But I wonder whether they could reduce the cost with smart software engineering (i.e whether they could compel themselves to do smarter software engineering by promising paid upgrades) or whether they could make a promise to make paid upgrades OR open sourcing the code for the components that will be different from the newer versions. (I.e. mainly the hardware drivers.) Yes, I know licensing issues may indeed pose problems for third party components.

Pretty sure the Custom ROM crew will leap on this one, so flashing it with one of those seems like it will be an option.

The long term practicality of this depends on mainline kernel support. LineageOS is cool, but the thing no one talks about is that while the Android stack might get updated, the kernel often isn't, which means any kernel level exploits remain unpatched after the support period ends.

Which is why this reads as kind of a stunt. A repairable phone is great, but if it becomes unsafe to use then what's the point? The software needs to be "repairable" too.

For how much? This is something that might make sense for flagship phones: getting two more years out of a $1000 phone is something people might pay for. For a $200 phone not so much.

In that case replacing the battery doesn't make sense either.