A lot of this feels a little "glutton for punishment". Like, they keep f*cking with him yet he continues to interact..
He just didn't notice when it became irredeemable because hey, he's been working with DOOM for decades at this point and it's always worked out.
Mick Gordon's been a household name thanks to his legendary DOOM(2016) soundtrack, so I'm not surprised he was trying to keep it up. A musician can really live or die by their name recognition and that open letter did a helluva number to his reputation.
couldn't he have sued on the basis his name was attached to it?
You have to prove that they're untrue and that they caused you harm.
In Mick's case, that harm would presumably take the form of lost income in form of work he didn't receive from other companies due to Marty's false claims. That's difficult (read: expensive) to prove in court.
Also, as others have noted, suing iD/Zenimax would also be some level of professional suicide. There are way more talented arts than there are paying artist gigs. That means game companies have hundreds or thousands of artists to choose from, and they consciously or subconsciously filter out the "risky" ones.
But when you're in the thick of it, involved with a project you really want to succeed, and which has an impact on your reputation, it is so, so easy to convince yourself that it's all a misunderstanding and you just need to keep trying. Particularly if you are a good-hearted person yourself. I've certainly suffered from this (and very few people who know me would agree that i need to be more of an arsehole!).
so why bother ever paying one?
Obviously, if the studios never pay, then nobody agrees to work for them, and the whole system of creative production just collapses.
Related note: iD is going to have a harder time hiring composers now that this is public.