Eh...Crispy Master looked that way, because he used up all his Regenerations, and his body attempted a Regeration, I believe. So, I don't think it's a problem, that there's a little bit left, just not enough for a full fledged Regeneration
That's how I saw it.
Realistically, it isn't that the 13th original incarnation of a Time Lord can't regenerate anymore... it's that it isn't advisable, because he isn't likely to survive if he does. The Master was a fluke in that he survived, but he didn't *really* because his 'fourteenth' body began to fall apart instantly. I think even the Doctor lampshades this by saying it's only the Master's sheer force of will that is keeping him alive when his body is practically the walking dead.
And then this '14th Master' spent the rest of his life trying to regain a way back to true life, only to find each method was only a short-term fix. Firstly by trying to harness the power of the Eye on Gallifrey, then by stealing the body of Tremas and rejuvinating himself via the Keeper of Traken, and finally culminating in trying to steal the Doctor's body, because he figured out that only a Gallifreyan body will do to truly sustain his life. Anything else, he realized, was impossible.
And then, the Time Lords resurrected him with a new life cycle. I wonder how many incarnations the 'new' Master had? We only saw a couple on screen, but I guess there was no saying that the YANA Master was the very first of his resurrected life cycle...