This is a very sore subject with me.
In the autumn of '17, I was contacted by Congress for recommendations pertaining to gap-filler radars. I did a great deal of work on the topic and provided a report -- at no charge -- to committee looking into it. You can find a few items that were extracted from my report, here:
Why We Need Gap-Filler Radars There was a
great deal of momentum at that time and Congress seemed primed to move. By law, the NWS was supposed to provide a prompt response.
Instead, the NWS sat on it.
When NOAA/NWS finally responded, to everyone's shock, the NWS told Congress gap-fillers would make no difference! The story is here:
https://www.washingtonpost.com/weather/2020/11/21/radar-gaps-weather-service/
Of course, it is preposterous that the spacing of radars is not related to warning quality.
Congress, in a time of huge deficits, hates to spend money when the agency in charge implicitly tells them it would be a waste of money.
So, gap-fillers are dead for the foreseeable future.