I think the micros are a good, cheap, starting point for people to get into the hobby.
The mini-z is good because for around $150 or less you can be all set to race. The only problem, at least where I'm at, is that people investing $150 aren't veery likely to race week in and week out. Maybe once or twice, but it's a fun toy to chase your cat/dog with or setup a small track in your apartment or house.
The 1/18's are another great starting point (or move-up point for mini-z's) for those that don't want to invest tons of money but still want to be able to tinker with setups and parts. It can definitely get expensive if you want it too but it doesn't have to. If more companies made 1/18's I think the racing market would grow but ultimately it'd lead to 1/10 investments.
I personally went from mini'z to 1/18 to 1/10 within 3 months. I bought the mini-z because I missed the RC stuff and just wanted to play around, but then the bug hit. I read about the M18 and for "a little more" money I could get that 1/10 feel I missed. And that worked for about two weeks before I just had to get myself another 1/10 car again.
I know I predisposed to RC since I used to have cars way back when, but I can easily see how someone who's never done "real" RC before take the same path. You hit a limit and then you want to go higher.
I think the Mini-T is a great idea too. I really hope it leads to Mini-BK2's.
But again, it'll probably lead to people getting bigger vehicles.
Which get's to the point that 1/10 will never die. It can't. Nothing beats the speed and air you can get with 1/10 vehicles. People want to go fast and jump high. That's why the nitro market is doing so well.
Ultimately I think the mini's and micros are just stepping stones that have been unfilled up untill this point. I don't think they will catch on big in the racing scene, but hopefully I'm wrong.