But that's obviously not what I was talking about. I wasn't even talking about parallelism in the context of language design. If an old tool is written in C and uses shared global mutable state so extensively as to make a performance boost from parallelism so impractical as to require a rewrite, then that's exactly the thing stopping people from "just" redirecting their efforts to improving old software. Because sometimes small incremental improvements aren't enough, and getting the buy-in to do a full rethink of an old tool takes tremendous social capital. And this is just about improving the old tool using its existing language---this isn't about going and using a new language with better support for parallelism. (Which, by the way, comes in many flavors.)
At no point did I talk about fancy parallelism related language features. At no point did I say that we should just go and add parallelism to an old program because that's what all the cool kids are doing. I framed it as a specific solution to a specific problem: making the old program faster. People like faster programs and speed ain't a fad. If you want to bring in examples of adding parallelism that don't improve performance, then great, but leave me out of it, because at no point did I advocate that.
I think your interpretation of tabtab's comment is a real reach, but whatever. Particularly given their other comments denouncing the evils of lambdas. Really? Give me a break.
I think what he was getting at is that the idea of languages integrating parallelism such that almost all code would be parallel all the time has proven to be a dead end. For the functional language research world promised for a long time that the main benefit for FP was automatic parallelism (no mutable states, you see). But it never really happened.
And in the more mainstream world, Java 8 invested heavily in parallel streams, but I've never seen an actual parallel stream in real code. I see threads all the time, despite everyone agreeing how awful they are. Parallel streams? Auto-parallelised FP code? No, doesn't happen. The level of parallelism is too small for programmers to think about. Most datasets are too small and most loops too unimportant to even take the risk of a bug creeping in through attempting to use parallelism, regardless of how convenient.