It seems like the punk aesthetic reached its zenith in the early 00s. That was the golden age of Warped Tour, after all and you had tons of pop punk bands on the radio.

This depends on whether we're referring to pop punk or regular punk.  If we're referring to punk as a whole, then 1994-1995 was definitely the genre's peak period.  Green Day's Dookie (technically pop punk, but still much more authentic than what would grow popular in the 2000s) sold millions upon millions of copies, and the Offspring's Smash became the highest-selling album ever from an independent label (which is especially significant, considering punk's anti-corporate morale).  Bad Religion, Rancid, and other punk bands also really caught on during this time.

Pop punk had its origins in the mid-90s with Green Day and eventually No Doubt, Sublime, and the Offspring, but for most of the 90s, the genre did not have a large number of bands that reached mainstream success, unlike post-grunge, alternative rock, and the like.  Even after blink-182 released Enema of the State, pop punk still did not yet garner much of a following, as 90s heavyweights Green Day and the Offspring declined in sales with their respective 2000 albums.  Everything changed, however, starting in the summer of 2001, when blink-182 released their second album following Enema of the State, and Sum 41 came out with Fat Lip, which proved the genre was significant for more than just blink-182 and the first wave pop punk bands.  Both Take Off Your Pants and Jacket and All Killer, No Filler produced multiple airplay hits, which were subsequently followed by Jimmy Eat World's mainstream breakthrough with The Middle, and eventually Avril Lavigne's Sk8ter Boi, both of which helped diversify the industry as new bands began to receive heavy rotation on MTV.  By about late 2002, pop punk was just as dominant and well-established as post-grunge.

While the early 2000s were definitely part of the peak of pop punk's mainstream success, I would personally argue that this plateau continued well through the rest of the decade, as well.  It was in September 2004 that Green Day released their monumental comeback American Idiot (which would grow most successful in 2005), which earned pop punk more critical and commercial attention than ever before.  Even with blink-182 haven broken up in 2005, Green Day was accompanied by several other pop punk and emo bands on the mainstream airwaves during the last few years of the 2000s, among them Fall Out Boy, the All-American Rejects, Boys Like Girls, Paramore, Angels and Airwaves, and Panic!  At the Disco.  It wasn't until 2008 that pop punk really started to decline in popularity, eventually dying completely after Orianthi's According to You.

Pop punk essentially went through two phases in the 00s.

2000 - 2004: Pure Pop Pop Punk Phase

A lot of the groups people think of when they think of 00s pop punk - Blink 182, Sum 41, Good Charlotte, and such.  The music had a rebellious, but fun image and didn't take itself seriously at all.  A very silly style that, whiled derided at the time, is probably needed now.

2004 - 2008: Pop Emo Phase

This is a controversial phase, especially since many seem to not be able to agree on what emo is, classifying the popular bands as pop punk.  Whether you believe that or not, there was still a stylistic and aesthetic difference between the early 00s bands and groups.  Groups popular in this phase include groups like My Chemical Romance, Panic at the Disco, Fall Out Boy, and others.  Of those, Fall Out Boy is probably the closest to the early 00s image, but still had things that differentiated them.

Punk peaked back in the 1970's with The Sex Pistols.

2004 - 2008: Pop Emo Phase

This is a controversial phase, especially since many seem to not be able to agree on what emo is, classifying the popular bands as pop punk.  Whether you believe that or not, there was still a stylistic and aesthetic difference between the early 00s bands and groups.  Groups popular in this phase include groups like My Chemical Romance, Panic at the Disco, Fall Out Boy, and others.  Of those, Fall Out Boy is probably the closest to the early 00s image, but still had things that differentiated them.


Whether there's really a major difference is debatable.  For one, Fall Out Boy hardly qualifies as emo, but acts like Paramore , Orianthi, and Love Drunk-era Boys Like Girls, who were popular towards the end of pop punk's mainstream run, fit better under the description you had for the early 2000s.  Also, emo was a significant mainstream force since 2002, when Jimmy Eat World's The Middle broke into the top 10 (a big deal for a rock song at the time), and even early 2000s bands like Simple Plan have a lot of that high-pitched vocal, emotional type of pop punk that you describe as having been the standard in the late 2000s (consider the song Perfect, for example, which was popular in early 2004 but came from an album released in 2002).

Was there much popular emo in 2004? I guess you did have Relient K and All-American Rejects, but 2005 IMO was the true breakout year.

2003 was the peak of pop punk in my opinion.  It started to decline in 2004 and lingered into 2005.  Personally I consider the pop emo that replaced it to be a different genre altogether, though there does exist some very close similarities.