I hope that the "majority of functionality" online will be online racing and downloading Scapes. The statement about the "online experience is 15% of the total game" is ambiguous at this point. Nobody knows what it amounts for, and we're not going to know until we get closer to release.
Major involvement with online connectivity in home console video games is here, and has been here since 2009, with no signs of decline and only becoming more widespread. I don't speak for everyone when I say this, but there is still a good number of people who don't have reliable internet connections for online connectivity in video games, either because their internet service provider is unreliable by default, or their living circumstances require connectivity methods that don't favor the best results in online gaming, such as wireless connectivity.
There is still an overwhelmingly large amount of users who have internet service providers with bandwidth caps; why these still exist in present-day is beyond me, but the people who have these services usually cannot help it, or are unable to find a better alternative. I mean, I've had other people (not on this forum, mind you) tell me that I should relocate and move out to another apartment complex just so I could play online gaming with them...
...yeah, as if I have the convenience of easily locating another apartment for sale/rent that is within the zone of another ISP, securing all of my belongings and finding a job immediately at the new apartment in order to sustain personal continuity, just so I could have an internet connection that is capable of sustaining uninterrupted and stable online play.
The development models of "release-it-now, finish-it-later" or "Game as a Service" for video games almost always relies on downloading additional content from the internet once the game has been installed. It's almost certain that Gran Turismo Sport is going to have a day-one update, either for enabling online connectivity and services, or adding additional content alongside that, but it needs to be downloaded in order for players to access it.
Those with unreliable or slow connections are able to get the content, but then their connection would prove to be unfavorable when they play online. Those with bandwidth caps would also be able to get the content and perhaps play online, but would otherwise quickly consume their bandwidth caps; they could also choose to decline downloading the content, but then they won't have the latest features at the compromise of conserving their bandwidth limits.
At this point, I'm hoping that the offline content of the game will prove to be satisfactory enough for me to enjoy, hopefully for other players as well. Gran Turismo Sport is essentially the "Gran Turismo 3 of this generation"; Polyphony Digital has a new platform with new technology to work with, and I'm certain they're eager to show players just what they can do with it. With high-speed online connectivity in home consoles making up a huge part of that technology, the outlook here could be a bad view for some.