I believe Kafelnikov will remain the last one to win both singles and doubles at the same event (RG 1996), but it wouldn't surprise me to see more players winning doubles either very early or very late in their careers and singles in the middle part.
Yep, no singles title contender will play doubles in slams. They have to win it before or after their peak. And I think long-time singles' top players are quite unlikely to concentrate on doubles at a lates stage of their career, they rather retire. They need to win doubles slam before singles success. But players who are singles top players only for a short period, are more likely to play more doubles later. They may be singles GS champs at 23 and doubles GS champs at their 30s.
But, singles is gotting more and more different from doubles all the time, e.g. the importance of net play is reducing in singles in favour of baseline playing. That means worse doubles players are winning singles slams, just look at Djokovic who's a doubles mug. That can make singles+doubles GS champs even rarer.
Does mixed doubles count? Because if Janowicz reaches full potential, and wins a Slam, I could easily see him winning a mixed doubles slam with Radwanska.
If he's to become a long time singles GS contender, he should do it ASAP before he won't play (mixed) doubles in slams.
But to OP's question. I believe there will be singles+doubles GS champs in future. Not many but some. But not in the same tournament.