And how would you necessarily guarantee that best Federer would defeat best Djokovic at Wimbledon? Both 2005-2006 Federer and 2014-2015 Federer had different strengths and weaknesses. I used to over-rate 2005-2006 Federer, but now, looking back at his matches repeatedly and comparing them to his 2014-2015 matches at Wimbledon, perhaps the decline is not as astronomical as some people make it out to be.
Let's compare their strengths:
Forehand - 2005-2006
Backhand - 2014-2015
Serve - 2014-2015
Net game - 2014-2015
Movement/Speed - 2005-2006
Slice - 2005-2006
3-3 in terms of skill set. Now, the extent of the advantage that 2005-2006 Federer has over his 2014-2015 self in some of those categories is obviously greater than vice versa, but 2014-2015 surely had more experience. Prime Federer, while he had all the power and potential in the world, was just a ballbasher who thrived against relatively weak competition. He had subpar problem solving ability, just relying on bashing winners past inferior opponents.
30s Federer, while he did not have as much power or youthful athleticism, is much more experienced, tactically wiser and excelled at constructing smart gameplan that got him out of vulnerable situations.
After all, as Master Wu from Ninjago once said "You assume your youth is a greater weapon than my experience."
Also, if we learned anything from watching Djokovic's matches, we as tennis fans should know by now that the Serb is capable of raising his level at moments when it really matters. This happened many times in his career. For example, it would be silly to think that he played at the same level against Federer as he did against Anderson in 2015 Wimbledon, or that he played at the same level against Federer as he did against Simon in AO 2016. Over the years, Djokovic has surprised many of us fans who have often underestimated him before big matches, only to witness flabbergastingly good performances the next day. Examples, 2019 AO final, 2016 AO semi-final, 2015 Wimbledon final, 2015 USO final, 2021 AO final etc.
If, in a hypothetical match, Djokovic were aware that he was facing a mid-20s peak Roger Federer and if he really sensed that he was under serious threat, how can we be so sure that Djokovic would just stand there and let himself get destroyed, instead of doing everything he can to elevate his level further to keep up with prime Roger? And we know how notoriously badly Federer struggles when he finds himself in clutch situations against his equals or even slightly inferior players.
Examples, 2005 AO semi-final - Fed had a match point in the 4th set against Safin, but blew it with some silly tweener. 2008 Wimbledon final - Fed had break points at 4-3 in the fifth set against Nadal, but couldn't convert any of them. 2006 Rome final - another 40-15 drama. 2011 USO semi-final - double match point on his own serve, but ended up losing 4 consecutive games after that.
So even if 2005-2006 Federer were to defeat peak Djokovic on grass, he would really have to close out the match in 4 or else, if they go to 5, his chances of winning become slim.