Any matchup that features somewhat similar styles can become tedious if dominates the sport to the exclusion of any others. That said, it's not the fault of Djokovic and Nadal that Federer is declining and nobody else is stepping up to consistently threaten them besides Murray (and how is that he writes this after a Djokovic-Nadal final and not an infinitely more tedious Djokovic-Murray final btw?).
And he's wrong to suggest that these matches are simply a matter of 'outlasting' the opponent with little skill involved. Djokovic and Nadal up their games at various points during their matches and whoever plays skilled attacking, though admittedly largely baseline, tennis on more points generally gets the win. The backhand smash dig is also odd as I have a crystal clear image in my head of Nadal's form when making that shot.
He seems to think that people don't respect players from past eras, which is a misreading of people who love today's game. It's quite possible to enjoy multiple eras for whatever high level tennis was dominant at the time. There's also a rather defensive argument that Federer is better to watch than Nadal and that that somehow lessens Nadal's achievements; reminds me a certain blogger that was just linked to on here. It's a completely subjective argument; I myself would find two Federers repeatedly playing for titles tedious, but I'd still respect their talent on its own merits and I certainly wouldn't write a whiny op-ed about it.