Is this a man thing, this obsession with quantitative comparisons between individual players and between 'eras'? What is an 'era' anyway? Which 'era' is Nadal in, which 'era' was Agassi in? These comparisons are not meaningful. In previous 'eras' the men's field wasn't as strong. Sampras would stroll through a couple of matches. So perhaps today the field as a whole (at least in mens tennis) is stronger, even if the Kei/Raonic group aren't.
Judgements are often subjective. Is Fed really 'over the hill?' I definitely thought Nadal was not the player he'd once been when they played in Oz, whereas Fed compensated for slower movement (was it really slower? I found it hard to judge, but perhaps that's just me) with improved forehand and his serve seems as good as ever - or better.
Weather and fatigue can contribute to the outcome of a match. After a very long match against Delpo Novak was exhausted in the 2013 Wimbledon final - plus both Nadal and Fed went out early. Cilic destroyed Fed at US in 2014? after Fed had saved match points in previous exhausting match vs. Monfils. Kei exhausted at US after very long match vs Novak, even if it was only 4 sets. Weather - Fed destroyed Murray on a very hot, dry day at Wimbledon 2015 in semis. Weather for the final was completely different, damp, heavy, leading to slower court and heavy balls.
None of these observations is an 'either' for or against any of these players, for example, Cilic might have beaten Fed anyway, as he nearly did at Wimb 2016.
The 'weak era' argument was originally dreamed up to belittle Federer, but now seems to have a life of its own. The 'slow courts' argument militates against Novak and his fans, but the truth is everyone has become fed up with the slowness. Even Kevin Mitchell of the London Guardian, who is no fan of Federer and even accused him of dying his hair to conceal his age, recently said that the slow courts and grinding tennis was 'strangling' the game a few years ago. Remember the blue clay at Madrid? Nole and Nadal were furious because it was 'too fast' (and blue).