It's not that the game is unplayable, or there's a DRM that prevents you from playing the game. the singleplayer events are still open. It's that the game had a heavy multiplayer skew and was all about online, competitive racing. So 80% of what made the game worth is is basically gone. It's the fate of an online game without dedicated servers that anyone can host.
Look at Quake. People still play that game because all you need for online is a computer that can create a server. There's no server in ID's offices you have to connect to, you can host one by yourself. If GT Sport let you do this,they'd never need to shut jack shit down because anyone with a PS4 can play. But because everything's on a central server, you're kinda fucked, here.
I'm pretty sure it works the same way for Mario Kart 8 and Splatoon, too. Since the game is P2P, you can still even play the old ass Wii U version online despite Nintendo basically abandoning anything that has to do with the Wii U.