The recent 1.10 patch added more single player content. The new single player campaign has 15 series and 45 new races. They have said that they will be adding more races in the future.

Do you think this is enough content for the people who mostly play single player?

It's a start. It definitely is enough for now. It's 50% off today on the EU store, so the sale and the update convinced me to get it. Sucks you can't save offline for the SP stuff, but I can get past that. The most important thing is that they already announced it will get more SP content in the campaign, along with tracks, cars and other stuff for all modes, of course. It's definitely a GaaS type of a product and so far they seem to be updating it pretty frequently. Plus, it does not have microtransactions and since release they added around 16 cars for free with a total of 50 free new dlc cars coming in Q1. I have no doubt there will be paid dlc in the future but the updates so far have been phenomenal and turned around my opinion of this game. Can't wait to jump in tonight and play around with the amazing livery editor aside from racing!