Yes it looks like it will mainly benefit new planes, but it in theory could provide downward pressure on the used market. If the cost of a new plane drops by a significant margin and more people can afford them, the cost of a used planes must follow suit if people want to sell them. In addition it may not help the STC market (although there seem to be some provisions for that area), but as far as new avionics, engine replacement, and even OEM replacement parts could see a price reductions. This would allow a greater number of people to upgrade and maintain existing planes.
Car makes are constantly pushing the new tech and leaving old tech behind. While in GA we have been forced by regulation to live in the past. Manufacturers are not going to want to lose what tiny market shares they have, but you may find them more excepting than you think once the ability to innovate is brought back into the market. As the end consumers we should welcome any small thread of hope change, but we may lose some old friends along the way. Plus, I fail to see how many of these small companies (by today's standards) would have that much sway at a federal level.
I want to be optimistic about this. In most cases when you reduce clearly burdensome regulation, the markets effected will get better, and many of the even more reluctant companies will benefit.