I like his "non-singing" though. Saying he cannot sing is a bit unfair, I'd say he has his own unique style that doesn't conform to standard singing techniques, and when it does come to standard singing techniques, yes, I'd say he's probably not very proficient at that.
But as it is, saying he can't sing at all is like hearing someone do guttural vocals and saying they can't sing at all. It's a different technique, and for Mustaine, he's very good at what he does. If he was TRYING to sing normally, and this is what we got, then yeah, I'd say he couldn't sing. And I haven't heard him actually tru to sing normally to make that judgment. But what he is doing, he's getting the exact result he wants out of it. And honestly, I much prefer listening to Mustaine's vocals than anything guttural.
But yeah, for his purposes, aside from the odd clean passage here and there, he really doesn't need his voice to be in any kind of an excellent shape, so I don't see why he couldn't go on doing what he's doing well into his 60s. If anything ever causes him to stop, I have a feeling that it's going to be his hands long before his voice.