GTA V, Rocket league, PUGB, Destiny, The Division , BF, Rainbow six, FIFA, COD, I could go on. I know almost all of these have a single player component but they keep charting because of their online component. The only purely single player game that can compete with these is Uncharted 4 and even that has a huge MP aspect.
I'm not an online guy but I can see the shift to MP and games as a service type games being the most popular this generation (not that there is no place for single player only games mind).
I only have experience with GTA V, (Star Wars) BF, and Rocket League, so I won't speak on the others. But GTA had a whole ton of offline content when it launched. And, perhaps most importantly: GTA Online gets regular updates. I agree that the online aspect is what has kept it selling for nearly four years (!!!), but I sincerely doubt it would have done so had Rockstar not constantly worked on it.
Rocket League is a slightly different situation. It's a team-based game, it's not full price, and it requires a relatively low level of skill to jump in. It too has received a steady stream of updates.
What I get from this is that these games need to stay fresh in peoples' minds with constant improvements/DLC. Polyphony has hinted at post-release support, support that mercifully won't take as long as GT5's to come on-steam. But we don't really know any other details. GTAV has free updates, but it has all those shark card microtransactions (something Kaz said won't be a part of GT Sport). It sold
substantially more than
GT pretty much anything, so I'm guessing that makes it easier to "write off" the constant post-release support. SWBF had pricey DLC. Rocket League has a lot of at-cost stuff too (though more reasonably priced).
The lack of a Season Pass makes me think there isn't a set schedule for GT Sport's DLC yet. It also means there's a chance it could be free. We'll find out soon enough...
...And the livery editor, and brand central, and the campaign mode (even if it's criticized for being pointless to some). I do agree that the online component is crucial though.
The livery editor looks, in ways, better and worse than other offerings out there. That being said, knowing how much time I've sunk into those other ones, I can see myself doing the same in GT Sport — I'll just run out of car-specific ideas far quicker!
From what we've seen of the campaign, it certainly doesn't look substantial in the same way its competitors' do. There's still more to see of course, but a bit over 100 training missions and one-lap-magic style events doesn't sound the same as simulated multi-year experiences in the motorsports world with contracts and invitationals. Or even what was possible in previous GT careers.