This is happening, because Project Treble still requires OEMs to ship updates and in one year after its introduction, Oreo got a meager 10% when Android P was announced and OEMs were still shipping Nougat with updates to Oreo, which doesn't require Project Treble compliance in such cases.

Google is finally facing the issue that regular consumers aren't caring to update anymore, specially now that devices are quite usable for several years.

So if they want to bring new OS versions into the market they really have to force OEMs to do it.

Hence the new GSI concept, finally adding update clauses to the Play Store usage agreement, and now upstreaming kernel changes.