I don't see Rafa doing well at Wimbledon since he hasn't done well there in forever. Too many dangerous servers/big hitters can take him out, and that's not even counting the good players. I think he's struggled on grass since his knees became sort of shot in 2012 ("shot" is a big exaggeration, obviously). Definitely affected his movement since and he struggles do go down low when he needs to and his movement has looked awkward on that surface for a while now. Not that he was ever fully comfortable but he reached 5 finals in 5 appearances, which is hugely impressive.
I recall you arguing that Nadal of 2010 was inferior to Nadal of 05-07 because he was 'slower' when his best years have been 2008, 2010 and 2013. Now you are arguing that since 2012, his knees are 'shot'. This year, Nadal has looked as good as ever, perhaps a bit below 2010 and 2013. Back in 2008, did Nadal make AO finals? he was decimated by Tsonga. His FO win this year was as dominant or more dominant than in 2008. This year, i have seen 0 evidence of bad knees or physical issues.
When it comes to Wimbledon, i don't think it has anything to do with his knees... I think Nadal has always struggled on grass. With the exception of 2008, he had to get past 5 set matches to get to finals, he has always been vulnerable in early rounds. The problem is not his knees, it's that Rafa enters grass court season after dominating on clay and getting too accustomed to clay, then in early rounds of Wimbledon, he is trying to adjust to grass when grass isn't worn out yet. To compound his problems, he runs into guys that do nothing outside of grass but on grass, are very dangerous. If he gets past early rounds, he is a different player and harder to beat... Given he hasn't done well in previous years, he has less confidence too so this makes him even more vulnerable in early rounds.