Ronnie's so fickle, that I wouldn't take any retirement statements seriously. He'll probably change his mind again at some point. Brilliant player (and it says something when his multiple world titles/final appearances are still an underachievement compared to his talent), but mentally all over the place from one year to the next. Hell, sometimes from one frame to the next!
Anyway, Bjorn Borg is probably the archetype for the trope of the LostChampion. George Best is another example, and so would be Bobby Fischer (if you'll allow games other than sports).
In all those cases - and with Ronnie - it's not so much that they lost motivation; more like they all had their share of inner demons that needed addressing in other settings.