Are radars still needed, or will radars start coming down soon?
There are still wide areas where ADS-B is not required and many aircraft without it. If an aircraft will only be flying in Class D, E, and G airspace below 10,000 feet, most of that airspace does not require ADS-B. Even in airspace where ADS-B is required, it's possible to get exemptions for non-routine flights without ADS-B. Aircraft without electrical systems are also exempted from some of those requirements.
These are basically the same requirements for operating with a Mode C transponder. But without a basic transponder, at least ATC can still get a primary radar return for a location. If ATC could only rely on ADS-B, they wouldn't get any location for those aircraft. It's probably not the best solution to just accept that these aircraft would be invisible to ATC.
And even aircraft with ADS-B can have systems fail, either ADS-B directly or something else that requires ADS-B to be turned off or lose input like position. It's also possible that the ATC side of the ADS-B system could fail. In these situations it's very helpful to have another system to fall back on.
Maybe you could argue that in areas where ADS-B is required the radars are no longer needed. But those are also the areas where it would be most helpful to have a backup method of locating aircraft. They are also still providing radar services outside the areas where ADS-B is required.
In order to get rid of radars, new rules would have to be enforced that require ADS-B in much more airspace.
Radars are still needed. Aircraft have failures. Pilots accidentally turn off transponders. Radars have weather modes and thunderstorms don't have transponders. Even flocks of birds can show up on radar. Finally, current rules allow flying in certain airspace without even having an electrical system onboard.
ADS-B is an enhancement providing more data and faster refresh, but it is not a replacement.