Only a few percent of people use open source router firmware, so the idea that it is getting axed because it lacks built in backdoors is pretty silly. Even if every router available for sale worldwide lost its ability to run anything but the 'official' firmware tomorrow, there are countless millions of older routers that will be available for many many years so those who want to do this will continue to do so.
Anyway, how do we know that DD-WRT and OpenWRT don't have backdoors? Maybe something unintended - many 0 day flaws turn out to have been in code for years before discovery, after all. Or maybe even something deliberate, if done subtly in the source or via corruption of the build machines used for the releases (google 'reflections on trusting trust' if you think that's not possible to do without someone noticing if they carefully audit things)
Heck, if the government really wants backdoors in routers, they just need to talk to Broadcom and Qualcomm, who make the chipsets for almost every consumer router.
Just doesn't seem worth the bother, since most people use Windows PCs, and Windows is a Swiss cheese of security holes. If the government wants into your Windows PC to spy on you, I don't think they have any trouble getting in through one of the many 0 days they surely have at their disposal. No need to bother with your router.