If only Sportmode and public/private lobbys would be online features, then i would say yes. But the game is so complex and "online" is even there where you don't expect it. To deconstruct the game and transfer it to an offline game will cost time, money and a lot of effort. If PD is willing to do that (if it's possible at all) is the big question.Let's say they turn the servers off next week, i reckon what will happen is an update will be released which allows for saving locally, if the servers aren't there to be used, then im sure this will be the solution.
Would it not be worth it for Sony to maintain a server for GT Sport funded by PS+ subscription?
And continue with the same deal as now, PS+ needed for online play but not necessary for offline. Extend the game life 3-5 years while people transition to the new game.
At the end, when there are no more updates save the data, livery, records on the Hard Drive.
I would assume that all cars in GT Sport will be in the next GT game. I'd also assume that the next GT game will have a livery editor. As little faith as I have in PD to ever do anything sensible or logical, deleting several years' worth of community content creation when it can still be used seems a bit foolish. I don't know the logistics of how the process would work, but I'd assume if the game uses the same assets them some of them would be able to be moved forward. I suppose it would depend on how drastic the graphical changes are for the next game.
At the very least, I'd hope they'd offer the chance to download saved photos and replays to the hard drive. I don't see why they couldn't, even if only for a limited time.