It's one of those things that you'd not go so far as to say "can't possibly happen"... but if I had to bet on someone breaking it and the option was "no-one" that'd probably be the one I'd go for.
Certainly if anyone does I'd be very surprised if it's not someone we've never heard of and maybe someone who hasn't even picked-up a cricket-ball yet (maybe even been born...) As has been said, it's just about impossible that a seamer will ever take that many wickets (it's astonishing enough that McGrath and Walsh have taken 500, there're unlikely to be all that many seamers who do such a thing). So it has to be a spinner.
Mendis, well, I don't think so somehow. He's started stupendously but it's inconceivable he won't to some extent be figured-out. Not that he won't go down as a very fine bowler indeed I don't think, but to expect him to play for as long as he'll require to take that many wickets, playing for Sri Lanka, is unrealistic in my view.
And as I've said before, there probably won't be another Warne-esque bowler for a long, long time, and there might very well never be another Murali-esque one. Any run-of-the-mill fingerspinner is obviously totally out-of-the-question unless we return to uncovered wickets (if we did such a thing mind you it could easily happen, but I don't think that's going to happen). So it'll require a once-in-several-generations bowler to do it.
Of course, Test cricket could have been destroyed by Twenty20 in 10 years' time so it might be a redundant question.