Except Kaz didn't say that at all and the headline of the article is not only misleading but completely false. Pretty click-bait article title, website is probably struggling for more attention/hits.
Whether you disagree with Kaz or not, if you read his quote he said is he considers Mission Challenge the same as the old GT career mode and they streamlined it into a new user interface.
“In a narrow sense, the part that was the GT Mode in the past is now called Mission Challenge,” he said. “It’s single player, there are various different types of races, from the very short to the very long, with different regulations, and you can tune up the cars that you purchased to participate in those races.”
“When people are talking about there’s no campaign mode, or no GT Mode anymore, that’s not quite true,” Yamauchi told us. “It’s just a change to the UI (user interface). All the things that you were able to do in the past in those modes are in the game.”
Instead of the familiar menu system, laid out like a map of the world with a home area that features your garage, as in previous Gran Turismo games, GT Sport features a horizontal menu with dropdrown sub-menus, much like the ones found on many websites and echoing the XMB (Xross Media Bar) found on the Playstation’s home menu.
This new approach will be alien to GT fans, but was necessary, according to Yamauchi. “Right now we have a very well organised menu and top menu, but if we’d placed all the features you see in the menus into a map format, I think they [players] would have felt it to be the exact same thing we had in the past,”
And in regards to "players not that into cars anymore," he thinks
new players aren't as interested in cars so they made the tuning settings more user friendly. He didn't make any correlation between axing a career mode and people not being into cars; in fact he never even mentioned removing the career mode.
"Twenty years ago, when we did the first Gran Turismo, people knew what happens when you change an air filter; people knew what increased compression ratios do to your car. But the new users don’t have that knowledge anymore because they’re not as interested in the cars, so we’ve simplified that area [of the game] so you can do the exact same thing you were doing in the past, but it’s more user-friendly for people just getting into cars.”