If there are enough customers that really want a headphone jack and everyone else was ditching them, it'd be a point of differentiation and hence worth those few cents for some manufacturer to offer one. I suspect that won't happen though because it'll transpire that most customers don't really care that much.

Samsung's still making a flagship line with a stylistic after pretty much everyone else dropped smartphone styluses; and they brought back external SD cards, too, which a number of manufacturers (themselves included) dropped from flagship lines.

It's possible that dropping headphone jacks could become universal, but I don't see any reason to expect it's likely.

> Which usually don't have OS upgrades beyond what they were sold with

Flagships usually do (Samsung flagships, and numerous phones lower in their line, tend to get two major version updates.)

> and Project Treble won't change that

If update practices are something a worthwhile market segment looks to in buying decisions, and Trello makes it easier on OEMs because their is a cleaner isolation of the device-specific pieces, then, I'd expect it to make more OS updates happen.

Good thing I can compile my own Android builds.