My guess is that Mark has (almost) stopped playing Sultans because "the twiddly bits" just got too difficult. He simply can't play it like he used to could, and thus decided to better not play it at all. Which is all fine as far as I'm concerned. Fingers do get rusty with age, there's no point in pretending that they don't, but luckily he has A TON of great songs that don't require guitar wizardry. Tracker will have more of those, and I'll be too busy hearing and admiring them to even miss Sultans for a second.