RCS reduction specifically targets X and S band radars because those are the ones which are precise enough used for firing solutions. You need those bands in order to either direct a missile onto a target or have a active seeking missile find it's target. What you're talking about is L band radar, which has existed since the 40's. But it's use is as a early warning system. The problem is that the resolution of it is orders of magnitude worse. For it to be actually useful today you need to build a massive radar which is very power hungry. That's a perfect target for a first hour cruise missile strike.

And that's not even getting to the part that the only reason why L band can even detect F35's and F22's is because that they have features like vertical stabilizers which are too small relative to the wavelength. B2's and B21's do not have this issue.

Finally the idea that one F-117 shot down somehow invalidates that fact that the F-117's stealthiness provided it protection with 99% effectiveness is absurd. The reason why it was shot down was because of USAF's arrogance in flying identical flight paths day on day and a SAM operator realizing this and succeeding in targeting it the moment it opened it's bomb bays, when it's stealth was degraded.