From my point of view, I think it all comes down to the players. Granted the ranking and penalty system could be improved but as some have previously posted, this is supposed to be fun. That being said, I think that might mean different things for different people (i.e bangers who just enjoy ruining it for others, some who just wants a clean race with people of the same skill level and those who just want to dust everyone else leaving them 1min. behind if they could). I hope that for the vast majority, that would be fair and clean racing with others who think and feel the same way.
I'm usually within 2-4 seconds of the faster guys and I'd like to believe that I am a clean and fair racer. I make some mistakes and always try to own up to them. Getting a time penalty or a drop in DR or SR rating for dirty racing is not enough. In real life, those moves would probably mean your race is over or you put yourself in a really difficult spot and you have to make up positions. With that thinking, in addition to the penalties, I think it would help to add damages to the cars to where it starts to affect their overall operating conditions. I would think that would deter some people from treating the game like Mario Kart. I've done about 110 dailies in sport mode at this point, I've had some awesome ones, some OK ones but a lot of crappy ones. Out of the 110, I've only quit 1 race (this past weekend at dragon trail). I started out 7th, I got taken out 3 times in the first lap and later got hit with a 10 sec penalty for a contact that happened while I was overtaking the other guy on the inside and he turned on me...at the end of said first lap...I lost it then and rage quit. From my perspective, I was on the inside line since he left the door wide open and I should not have been penalized since the other player turned on me.
All things considered though, I try not to let those moments ruin it for me. While I don't like online playing because I hate dealing with people childish behaviors, I love racing and GTS is literally the only game that I play online (bought PS Plus specifically for that) so I will put up with it...."for now". I figured it would be alright as the game evolves over its lifetime. I enjoy beating the AI just like the next guy but after a while its just boring so having a nice battle with someone else who races clean and fair is totally worth it IMO.
On a side note, I noticed a comment about slower drivers needing to let faster ones pass...I don't agree with that as it depends on the situation. When Lewis and Vettel just recently dropped back on the grid at the Mexican GP, the back markers didn't just let Vettel and Lewis pass...heck Lewis spent almost 15 laps behind Wherlein before he could overtake him...If they are being lapped then yeah, that's another story and they should pull to the side and let them pass. However, if they are racing for positions with the faster drivers trying to make it back up the grid, they should race for it IMO. I have seen that happened on numerous occasions where someone on the front made a mistake or got taken out and they fell way back. I don't just let them pass when they come behind me, I made it so they race me for it. In some cases those were the highlights of my race because those guys tend to be clean and fair as they make their pass. On the other hand, I've had quite a few run ins with those who became extremely impatient with me on the racing line, flashing their lights and such and ultimately rammed me off track just so they can pass....and that's not cool!