That's a great example of false logic; it doesn't actually prove anything.
For instance, in 87, Chris Evert lost to the French runner up, Wimbledon winner, in 88 to the winner at the Australian and US Open and runner up at Wimbledon, and in 89 to the Wimbledon winner (didn't play AO or FO). Was she in her prime?
For three straight tournaments, the 73 US Open and 74 Wimbledon and US Open, Ken Rosewall lost to the tournament winner (Newcombe in 73 and Connors in 74); was Rosewall in his prime because he lost to the winner at three straight Slams?
In 1990, Lendl lost to the winner at Wimbledon and the US Open, in 91 to the Australian and US Open winner (skipped Paris, lost early at Wimbledon), and in 92 to the Australian and Wimbledon finalist and the US winner; was Lendl in his prime?
The huge difference is these players were in no way having their best results ever, they were still inferior results in everyway to in their primes years. In 2004 and 2005 Hewitt was more consistent in slams than he ever was, including while he was #1. In 2001 he lost a slumping unseeded Moya in the 3rd round of the Australian Open, Escude in the round of 16 of Wimbledon. In 2002 he lost to Alberto Martin in the 1st round of the Australian Open, and in 2003 while still ranked #1 to El Aynaoui in the round of 16 at the Australian Open. In 2004 and 2005 he lost in all 7 slams he played to the eventual winner, 5 of those 7 to Federer, 1 to Safin, and 1 to Gaudio on Hewitt's worst surface by far (note I didnt even bother mentioning his round of 16 loss Canas in 2002). His only round of 16 loss was to Federer due to starting a year with a low seeding/ranking after an injured plagued 2nd half of 2003, and his only quarterfinal losses were to Federer at Wimbledon 2004 (more bad draw luck and a carryover of the late 2003 situation) and Gaudio at the French Open.
Yes Hewitt was definitely in his prime in 2004 and 2005, in fact it was probably his best tennis ever, but the field was simply better than the transition era of the early 2000s with Federer, Roddick, Nalbandian, and others in their primes now and Nadal emerging, yet he still handled everyone except Federer for awhile.