There is a bigger picture here. Realistically the CHEAPEST RC tracks for someone to set up are offroad and dirt oval. Carpet requires a large indoor facility and in order for it to be viable, it needs to be in a metro area which means it is going to be very expensive to procure.
Asphalt is easily the cheapest since you can set it up with fire hoses in a parking lot for next to nothing BUT the facilities capable of running big races are permanent which means $$$s for materials (paving isn't cheap).
So that gets back to the logical "if I am going to set up an RC track what is the most cost-effective type to build" deduction of offroad. Fill dirt is cheaper than asphalt by a lot. Offroad is fun and it can cater to a large array of racers from the beginning RTR truck hobbyists to the hardcore 8th scale racers. If I owned a hobby shop and wanted to start a racing program it would probably be offroad. Certainly I would try to set up a basic asphalt race series in a parking lot to get all the RTR Traxxas/Associated/Schumacher guys out having fun but the money would go into an offroad facility.
So if you had a local offroad track to run on and you wanted to race there the 1/8th scale buggies will probably look the most exciting and where a lot of people will choose to spend their money. Is 1/8th scale taking over? No. Is it huge? Yes. Will it get bigger? Yes. As costs of the buggies come down and the costs to be competitive in electric go up you will see more and more popping up.
I raced 1/12th scale and electric TC for YEARS. Being competitive simply got out of hand from a cost perspective. 20 years ago when I got my first RC car I never thought I'd say that racing competitively in a nitro would be cheaper in the long run than racing electric but today it is true. Sure a 1/8th scale car (on or offroad) is more expensive, but it isn't too terribly far off the cost of a high end TC roller. Engines are only a little more than a decent brushless system but then add in battery costs, charging equipment costs, etc. $100 for a starter box and glow igniter is a piss in the bucket compared to a Turbo 35 GFX and enough race packs to make it through a day.
I think TCs were HUGE for about 6 years but they have simply gotten too expensive to race now and a lot of people are going elsewhere. Nitro seems to be where the cost-benefit ratio is the best for us average guys.
The only form of racing I have really seen die in my 20 years of involvement has been 4WD electric dirt oval. Everything else has always been there and will probably always be there.
EDIT: Please note that I used to live in an area where we had carpet facilities. Where I live now the only tracks within a reasonable distance are offroad so cost isn't the only reason I am no longer racing indoor electric. I'd happily race 1/12th scale again if I had a facility that was closer than 2 hours away.
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