In 2003, Cohen faced a resurgent Kwan. In 2004, Shizuka showed up with triple-triples. In 2005, Irina resurfaced with triple-triples and the Russian federation hellbent on handing her an Olympic gold medal. In 2006, it is true, Sasha faced no real competition at Worlds and blew it. At the Olympics, however, she was up against the current and former World Champions, both of whom had better (or in the case of Irina, more consistent) jumping skills. So I wouldn't say she peaked in the weakest quad or even that she failed in that time period, since she was on the podium more often than not.
I also would dispute that Sasha couldn't have competed in the more recent era. She had a lot of things that were rewarded in COP, like her spins, and falls--her nemesis--are less fatal now than they were under 6.0. I think a top form Sasha would have done very well against Joannie Rochette, Carolina Kostner, Miki Ando, etc. So she still would have gotten a big medal haul of the silver and bronze variety, just like she did in the previous four years.
Kwan at the 2003 Worlds was beatable. She was great but it wasnt the 1996-2001 Kwan anymore. She had 6 triples, scaled down choreography, no difficult combinations. Sasha with a triple lutz-triple toe planned and Tarasova choreography would have beaten her for sure if she had clean, but of course going clean is a foreign concept for Sasha. Shizuka skated an amazing long program at the 2004 Worlds but she was a nobody at the time (and her short program was mediocre). The judges would have given Sasha the gold had she done something even reasonably good, which of course that night she didnt. Yeah Irina at the 2005 Worlds was unbeatable. 2006 Olympics the way the actual skating went the gold medal was waiting for her and she again blew it.
As for Sasha doing well over the last quad vs those skaters, I have never seen Sasha skate as well in any major event ever as Miki Ando did at the 2007 Worlds or even 2009 Worlds. Miki's jumps alone would have given her the edge on Sasha. Joannie Rochette is alot better than Fumie Suguri, and Sasha all those years had alot of trouble with Suguri and often lost to her in major meets (2002 Worlds, 2003 Worlds, Goodwill Games, 2004 GP final, 2006 Worlds), so Rochette who is much better than Suguri would have been an even tougher opponent for Sasha at her peak in 2009-2010. Mao Asada blew Sasha away in their first meeting when Mao was an underaged 15 year old (2005 Trophee Lalique) so imagine Sasha being competitive with a prime Mao and her peers. Mao the underaged junior was also 5-1 against Sasha, Irina, and Shizuka combined in 05-06 giving indication how the skaters of that quad would compare to the skaters of this. Lets put Sasha's performances from 2003-2006 into 2007-2010 instead:
2003 Worlds- 4th in 03, probably would have been 5th in 07 behind Meissner who atleast skated a clean short with a triple lutz-triple toe (Sasha fell on a triple lutz-double toe attempt in the short). Considering she fell on both a triple toe and a camel spin too in the long I might be being generous but I dont remember Nakano and Meier's too performances too exactly.
2004 Worlds- her 08 long program performance was blah and wasnt smartly loaded with content like Kostner's to compensate the mistakes. Probably 5th behind Nakano.
2005 Worlds- her 09 performances were ok but no triple-triple tries in either program and some small mistakes in both programs. Probably 5th behind Kim, Rochette, Ando, and Asada.
2010 Olympics- of course she won the silver in 06. I am sure we will atleast agree on there isnt a prayer her 06 performances would have won a medal here. 5th behind Nagasu if she is lucky.
In the 07-2010 quad her new nickname would have been Mrs. 5th place.