isn't optimising the games extra costly?

These days compilers can optimise it for the hardware mostly on their own.

I can imagine the annoyance of getting bad reviews on steam because some Dingus is trying to play the game on a 10 year old PC.

Yeah, that's the great thing about PC. You don't have to upgrade your hardware more than once a decade, and you can give feedback to games publishers that chasing ever increasing graphics trends is alienating their customers. You console gamers have to take whatever slop you're given, but us PC gamers don't have to worry about a publisher not supporting backwards compatibility, so we have more market power. We can apply greater pressure on the industry to apply pro-consumer business practices.

Also, something like 30% of PC gamers pirate their games

That's definitely not true. I wish it was.