I suspect it might be less easy to detect really stealthy stuff than what you are saying. While TODAY modern RAM and geometry cannot, for example, shield against S-band radars too well, the B-2, because it is entirely tailless actually doesn't show up on S-band much at all (the F-22 and F-35 do, quite well actually). So, I'd expect that by 2300 AD or whatever that we'd have pretty good stealth! Of course things like 'quantum radar' are likely to mean it will be a contest between sensors and stealth, much like it is today in the naval and aerospace regime. I guess positing that the sensors won that contest decisively is a defensible extrapolation. Still, I think it is likely to be quite possible to have small, stealthy, 'things' that can come at an opponent and get pretty close before detection. A lot of this will likely be due to the fact that active sensors basically broadcast your location, so their use will be pretty limited. Even today in an actual all-out battle ships and aircraft try to rely on passive detection until they make contact with, or are pretty sure about, the existence of an opponent within firing range that they want to attack or must defend against. Only large units that presumably cannot really hide, like carrier battle groups, would be constantly running search and fire control radars, as their general location is probably well-known already.