That is my point about the 325i Sport. It wasn't an exceptional car objectively and didn't have a motorsport pedigree, like the E30 M3 had. It was just a run of the mill 325i with better front seats (pull out knee supports like the RS Turbo Recaros had) and a few bodywork changes. What gives that particular example more merit is the fact it's a 2 door, which wasn't all that common back in the day.
Absolutely the MK5 was groundbreaking in the context of its predecessors. MK3 - awful. MK4 - awful. The MK5 was universally applauded by motoring journos and the buying public alike. It may not deserve the same merit as the racing specials like the E30M M3/Delta S4/UR Quattro, but it was definitely a revolution as opposed to an evolution. First GTI to get fully independent suspension, and the MK7/8 still uses the same basic underpinnings to this day. First GTI to get a turbo. First mass produced direct injection engine, which has won 'engine of the year' a few times. It also harked back to it's great Granddad MK1 with the tartan seats and Golf ball gear knob. These things matter.
Classic status is a complicated set of 'rules' and emotions not set in stone, but motoring significance is one of the measures the 'judges' commonly use. I'd say the MK5 was pretty significant and arguably restored the GTIs credibility in the hot hatch arena.
EDIT: Second GTI to get a turbo, lol. But the UK GTI initially got a crappy 8V 2.0 n'asp engine.