If Ronnie truly wants to, like you said SF he can still be winning titles well into his 40s because his natural talent allows him so.
One of the first aspects that starts to fall apart when you start reaching 40 is your long potting. Amusingly, Ronnie between 2006-2011, apart from some exceptions, lost his long potting ability and he wasn't confident at all on long pots. He regained that back in 2012 and that Higginson match at the German Masters wheh he was 4 nil down changed evrything, I would go as far as to say that had he lost that match he may not have won 2 consecutive World Titles, that match changed everything.
But on the matter at hand, Ronnie is far from finished, and it isn't just because Ronnie lost a match against Williams from 3 nil up that he suddently starts to decline as some (cough cough AC or LT) have suggested
That is just pure nonsense, those were the very same people that said Ronnie was finished in 2011 and what did he do.... he shut them well up and made them eat their own deluded words.
I am not naive though and say age isn't a factor, of course it is and decline happens to everyone, but like I said Ronnie's natural ability plus winning matches without losing much mental energy has helped Ronnie over the years in wining titles.
His long potting may start to decline in the next couple of years, but his breakbuilding, which is by far his greatest strengh given he's the best breakbuilder ever, won't fade away that easily, he'll break Hendry's record of tons this season, and I think he can still win one more World Title for instance and hopefully another Masters and UK Titles.
He now only competes when he truly wants to, in his mind he isn't "forced" to compete so he gives it 100% everytime he goes out there, if he loses then so be it, like in this match against Williams. He played poorly most of the match, but it wasn't that kind of silly-fast-don't care about losing performance. Everyone has a bad day at the office, and yesterday it was his.
It's nothing new, every year when Ronnie loses an important match the haters come out in force saying he's finished
I've heard that rubbish talk since I started watching Snooker... Those were also the very same who for instance said it was a change of the guard when he lost to Judd 6-2 at the Masters 2012...
Like I said, decline will inevitably happen, but Ronnie will still be around winning a few more titles along the way. And also, like SO said yesterday, he long ago doesn't have anything else to prove to anyone, he defended his World Crown in 2013 having a year out, won more titles, and from where he was in 2011 to where he is now everything is just a bonus really, I don't care if Ronnie retired tomorrow, he doesnt have anything else to prove, but since he's still around playing long may he continue to win a few more titles and gives us some of his magic Snooker.