It's like anything else,supply and demand.
Mk 1's went up because of scarcity and status
Mk 2's went up because enthusiasts were priced out of the MK1 sector.
Mk 3's will go up in time and already are but all models will only achieve a higher price based on condition,mileage,service history,spec etc.
Mk 4's will also go up in time but expect a very long wait as there are so many still around and so many were made.They were produced at the beginning of the "throw away" era.
The more there is of something the less it is worth.
Mk 5's ?. Lol,not in my lifetime and i'm only 3 years old !.
The answer is in how many were made and how well they were made.Quality went down with every model made.
Mk 1 suffered from rust which is why many were scrapped,they were also made in lower numbers compared to later models.
Mk 2 and early mk 3's suffered from rust in some areas.
MK 3 got better the later years they were made,same as some MK 2's.