Linus doesn't so much say that parallelism is useless, he's saying that more cache and bigger, more efficient cores is much better. Therefore, increased number of cores at the cost of single core efficiency is just stupid for general purpose computing. Better just stick more cache to the die, instead of adding a core. Or that is how I read what he says.
I'd say, number of cores should scale with IO bandwidth. You need enough cores to make parallel compilation be CPU bound. Is 4 cores enough for that? Well, I don't know, but if the cores are efficient (highly parallel out-of-order execution) and have large caches, I'd wager IO lags far behind today. Is IO catching up? When will it catch up, if it is? No idea. Maybe someone here does?