1) GT has been at risk of hacking since... forever. Even before the days where everything was stored on a nice, easily accessible hard drive there were cheat devices that made it rather easy.
2) If you actually sat down and played some games on PC you would quickly realize that the only games that really get messed with via mods and "hacks" are ones where the developer allowed it either intentionally or via incompetence. I've played plenty of games on PC where there was no modding community solely because the game was locked down like a bank safe.
So, which one is it?
I recall hearing this quite a bit when it came to arguments against having a livery editor in GT. It was a terrible argument for that and it's a terrible argument against modding. I've done quite a bit of modding and really the only "bad" content I've seen in any sizeable amount was low quality content (as in, visually low quality). That's not to say the content you're talking about isn't out there, just that I've yet to run across it when it comes to the games I play.
Although, as I stated earlier it's not even a risk developers have to take if they don't want to. They can even make certain aspects moddable while locking the rest away if they want to allow things like custom liveries made in a third party program.
Everyone has been at the risk of dying since... they are alive. What a great argument, it can applied to pretty much everything in existance.
Anyway, there are highly popular games, with highly competitive eSports modes on them, made by hugely experienced studios, that still didn't solve the cheaters problem. Offline games barely bother with that, as long as people pay for the game, it's all proffit.
But you will surelly pull up an argument that Battlefield, COD, CSGO and so on are/were never plagued with cheaters on PC. Literally the day after I wrote the post before that one, a Youtuber posted a video of a cheater on BF5, headshotting everyone with a sniper on his path. Everyone on that lobby must have had great fun!
Which one what? Yes, GT5 was hacked on console, and so what? It's a lot rarer to happen on a console game, than on PC. If GT5 was hacked on PS3, the same would happen on PC, and probably a lot sooner. That surelly was a lesson to PD.
Livery editor is quite different, don't you think? You won't change the performance of the car with a new livery (except stripes, those do make the car faster, as everyone knows!), and it doesn't affect the other players. And it's probably a lot easier to filter "offensive" liveries, than cheaters.
Cheats, will affect others on an online lobby, like it or not.
As for mods, they do make sense in offline games, in online games it all depends. I don't want to see cars created and used in online lobbies with fake/made-up stats. I don't want to see PD stop modeling cars with the quality they do, because "modders will do that for us". But I wouldn't mind PD allowing some creative freedom when it comes to creating tracks by the players. And that could be achieved on console anyway, if Sony allowed.*
Is this a must? No, PD has so much more to worry about with their new game (AI for example).
*PD scrapped Standard and Premium cars alike for GTS, due to their "lower quality", yet they would allow mods to include even lower quality cars compared to GT6 Premiums? That would be a strange choice. The same for tracks, if we were to get lower quality tracks from modders, might as well allow every track from GT6 to GT7.