I don't think NASCAR will be very afraid of the RTA, but it does raise some interesting points when you ponder the hypotheticals.
The RTA will obviously know that the fanbase is clamoring for more short tracks. NASCAR has a stranglehold on intermediates and superspeedways, but the RTA does have a bargaining chip in that there are numerous short tracks that they could partner with like
@AndyMarquisLive is saying.
As far as the cars, team owners have already said that the next gen isn't as cheap in actual usage as what was pitched to them originally. So they could tell NASCAR that they could build their own stockcar design, a more traditional late model style chassis, cheaper than the NASCAR Cup car anyway. Another win for them.
For prize money and exposure, to Andy's point too, if you lose the stars then it's game over. If all of the big teams and drivers were to leave, then basically the current Xfinity midfield would become the Cup series overnight. Fans would not stick around for that.
And then yeah, demographics. The NASCAR total viewership is still higher than F1, but their total viewship is also destined for a sharp decline over the next few years due to that aging demographic. NASCAR's fanbase is literally dying out and new fans from younger generations are not queued up to replace them.