Everything is review and opinion driven these days. Popular Youtubers still highly rating the MK5 kind of props up values to a degree, because they make it apparent the window of improvement is so small between MK5 - MK7.5. And I can attest to that having driven them.
Only the MK8 has moved things on a little, but even then, most of what you see underneath and under the bonnet harks back to the MK5. But let's not talk about the MK8's chintzy 'on trend' tacky interior accoutrements and questionable exterior styling
Haptic buttons and porn parlour LED lighting, I mean, WTF? When did cars become rolling wine bars, ffs
VW really did set the bar with the 5. I'd even go as far as saying they overbuilt it. Not in terms of the rust and overall fit and finish (as both are pretty shat tbh) but rather the driving dynamics. It's a difficult platform to improve on in a night/day fashion. The 5's steering feel is leagues ahead of the 7's. Haven't driven an 8 so can't comment on that one, but well done VW
That's partly why the 5 is holding it's value. It marked the turning point from the utterly dreadful MK3 and MK4 GTIs and it still looks good today. People still rate them.
I wouldn't pay that for that example personally. I'd probably go for a MK7 for it's better rust protection and overall fit and finish, and try to make it drive like the 5