I've bin riding for just over two years started on a black and gold t-rex, changed to a t-pro, then to a new t-rex which has almost everything upgraded, I am still crap though compared to a lot of people i ride with.
Your idea for a bike would last a long time for a beginner the rear hub would need changing eventually though. A twenty inch would be better for learning on but you should try and have a go on a few different bikes before you make a final descision.
I dont think it is possible to say, there is so many good riders these days, and there will always be someone pushing the boundries even further. I think there is just going to be a steady flow of really good riders rather than one rider standing out from the rest.
The new Echo Team rides really well and is very nice on the back wheel. I think you should have a go on a few different bikes and see which you like the feel of.
dont get try-alls absolute crap! onza ronnies are the best rim, super strong and hold a grind for ages i have an echo cnc rim wich is quite good and holds a grind well.