I don't think the bolded is actually true. And I don't think most of us do. Often, with elite players involved with lessers, there is a clear winner because one is a superior player. But when two top players are involved at the same time, it's more likely a question of the better on the day. In many cases, the loser doesn't actually "suck." As you say, there are shades of gray. The notion of "sucking" is a dismal performance. What I saw from that match was not dismal from Federer or "sucking." He just played less well.
I didn't say he was "fine" against Del Potro. I said that I didn't see the obvious signs of back injury that I was expecting, based on the report from the front, and the post-game later. And I have watched Roger play when it's clear he's compromised in his back. It's not hard to detect. I know Darth's script chapter and verse: 'how could he have gone 5 sets via Tiafoe,' and 'Youzhny...please?!' I'm actually willing to believe the Fedfan assessment that he was a bit compromised in the back. What I don't buy is that he was "rubbish." He was rather better than mediocre, by his standards. He wasn't stellar, and I know you all expect that. As to the the comparison with the 2009 FO and Rafa, it wasn't even in my mind. You're hanging onto that just a bit too much, my friend.