Luckily for consumers Android is a diverse ecosystem not controlled by one single manufacturer. There are many devices with headphone jacks to choose from.
What makes you think the other major manufacturers aren't going to follow suit?
It may just be a matter of time. Especially if 2 major companies normalise it, then all the other companies get to drop a feature and save a few cents on every phone.
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.
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.
Which usually don't have OS upgrades beyond what they were sold with, and Project Treble won't change that, because OEMs are the ones expected to ship OS updates.
> 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.