It will be interesting to see what happens. I'm kind of disappointed for selfish reasons, as I'm not finished buying the things I want, and what he's done at Saint Laurent is exactly the type of clothes I like to wear. I'm quite surprised with him having just re-launched couture and seemingly having as much freedom as possible.
What do people think of the future for Saint Laurent presuming he goes? If the brand is smart then they will keep producing all of the classic basics designed under Hedi (I'd happily continue buying this kind of thing at least, as long as it's not changed for the worse). I would presume most talented designers wouldn't be too happy to try to follow in his footsteps, and would want to put their own stamp on things. If that's the case, then if they're smart then they'd open some kind of access to order from his archive as a separate line, or at least bring back a selection of pieces from it each season.
It will also be interested to see what he does at whichever house he moves to (I presume Dior). Will it be more of the same? Either way with more options for us as consumers it can only be a good thing, assuming price points aren't too astronomical.
Would be sad to see him go, but if he goes back to Dior I'm excited. Much higher quality wise and would likely be continuing his current work with Dior as a reference again.