Android One has at least helped expand the options for phones with no crapware and a reasonable update policy. A decent number of them still have jacks.
Project Treble makes updates easier for OEMs but in no way guarantees that they will actually happen.
The Moto G6 for example has Treble but Motorola has said it will only get one major update, to Pie. It came out in May with March security patches, they released the May security patch in July, and then released the July security patch in September.
Android One devices will (supposedly) all receive 2 major version updates and 3 years of monthly security patches.
But when Treble is installed, isn't installing a ROM with the new version a simple business, at least for people who can follow technical instructions ?
As long as the bootloader is unlocked the ROM flashing process isn't really any different from non-Treble devices, at least from the user's perspective. It should make developing the ROMs significantly easier. Obviously there are Treble devices whose bootloaders can't be unlocked and third party ROMs are still impossible on those devices.
In principle a Generic System Image (GSI) can be flashed onto any Treble device and everything will just work. From what I've seen so far this is not really quite true and there will still need to be device-specific tweaks to ROMs, but the amount of device-specific code should be far lower than before and make life easier for maintainers. Hopefully the end result is more devices with functioning and well maintained ROMs available.
LG G7 One
Xiaomi A1/A2 Lite
Moto X4
Nokia 6.1/6.1+/7+