Personally I think Fiat made a mistake in producing the modern 500 range, all comments are negative about the car, older ones look tatty and age quickly.
Well, Fiat might say it's made them a lot of money, and has sold particularly well in the UK. Without it, it's entirely possible Fiat would by now have pulled out of the UK market altogether. And many folks would say it's still the prettiest car in its class.
I see posts here every day from 500 owners who are still delighted with their cars, saying it's the only car they've had in a long while that puts a smile on their face every time they drive it, so I can't accept your statement that all comments are negative about the car.
That said, Fiat have milked the 500 for all it's worth, and it's an old design now, getting older by the day. They've used it as a cash cow, rather than ploughing the profits back into designing its successor.
I'd agree with your comment about ageing quickly; IMO an older 500 could prove expensive for anyone without DIY skills. It's built in a modern factory and assembly quality is reasonably good, but it's built using some of the cheapest parts on the planet, and this shows in the number of posts we see regarding prematurely worn parts. A well used dualogic in particular is basically a liability waiting to happen.
The secondhand market doesn't reflect this, at least not yet; residuals are still amongst the highest in its class. But I think it's only a matter of time before reality kicks in. It used to be good value for those looking to pcp a new car every 2-3yrs as the sky high residuals kept monthly payments low and you'd most likely be rid of it before the real problems start, but pcp quotes have been creeping upwards recently, most likely because of concerns about end of agreement residuals.
Paying main dealer money for a 3-6yr old 500 could easily prove to be an expensive mistake. Folks in that market wanting a Fiat would do better to look at the Panda, where prices are more realistic. And paying £4k+ for a 10yr old 500 is way beyond stupid, but a quick look in the classifieds will see many priced at that mark.