Forgive me, but this "none of the top players are exceptional doubles players these days though." is a load of crap.
If top players gave any importance to doubles, they would clean up at every tournament they've entered.
Once let's say Federer and Nadal decide to team up and take things seriously, I don't see any pair on tour which would stop them.
Federer and Wawrinka regularly get their clocks cleaned by doubles specialists in DC, and they're two very good singles players who play a fair bit of doubles by modern standards.
IF the top singles players took doubles seriously, concentrated on perfecting the skills required, developed their tactics, built their teamwork, THEN they would possibly be quite good. But the point is, they don't. Nadal's net game is regarded as pretty good for a modern singles player, but by doubles standards it's below average. The less said about his transition game, the better. And he is miles superior to Djokovic.
Doubles is a different beast to singles, even more so in the modern game than 20 years ago.