When we buy new bikes, we are sensible and pragmatic - we look at value for money and reliability and parts supply and all the rest.
When we buy old bikes, money is less of an issue and certain bikes develop a legend around them (that's often overstated). For a lot of us, actually going fast isn't as important as it is when we buy a modern - instead, we go for the bikes that we wished we could afford/give us the horn/whatever.
If the "legendary" Maico 490 was really that good, then every serious privateer would have bought one in the day. Even now, they don't turn an average rider into a moto-god and are not essential equipment to win VMX races.
People have been going nuts over post-VMX KX and CR500s - I understand the appeal, but there's a lot of nonsense spoken about them too. There's a number of bloody good reasons why they stopped making 500s... (And I've got a KX5 that I love - but I'm not kidding myself that I'm fast on it).