Agreed. Staying within the white lines is well within every driver's ability. Although I think it would be better to design a "penalty" into the track (gravel, grass, etc.).
Its easier to design a penalty into the track than just brake a bit harder or turn a bit more? Grass or Graveltraps are hard to design since so many things like safety, other series need to be taken into account. Also it reduces the kind of corners you can do if white lines arent worth anything
It's not necessarily easier to design in a deterrent, but I think it results in more entertaining racing. All things equal I would prefer to not watch racing "evened out" with time penalties. You're completely right about it not being feasible in every corner though.