Too quickly for ATC to see the pop-up on radar?
That is exactly the point!!! Trying to have a human air traffic separation specialist still hand carry "radar vector" airplanes 1:1, with duplex voice, based on outdated or missing data that can never be real time at any feasible fully allocated cost, with a backup of "see and be seen" with a high speed jet flying into into a hazy day sunset, trying to see a C150, or LSA, or glider, or now 3 ft wingspan UAV,... is just nuts!
Current US ATS is pure and simple obsolete for any modern requirements and safety expectations from LSAs, to parachutists, to gliders, to A380s and F22s. The cost per unit separation service is at least one order of magnitude if not more orders of magnitude higher than modern technology with RNP, data links, and automated conflict probe and resolution now readily permits (IF WE HAVE THE PROPER CRITERIA and benefits allocation PUT IN PLACE).
Further, the airport runway capacity per unit time, per acre, per dollar is abysmally low, even at our busiest facilities like KORD and KATL, especially in convective WX. We can do much better, safer, and at lower cost, with many of the CNS tools already at hand (like 3D and 4D RNP, data links, RTA, and automated trajectory separation with conflict probe and resolution), if they're just combined and used the proper way (but NOT as in NextGen).
Now with the advent of UAVs, it is critical that ATS be rethought from first principles, ...just as the phone companies re-designed the phone system in the '50s, and as cellular technology displaced land lines.
Not only is the obsolete ATS configuration that facilitated this sad F-16/C150 event obsolete, but even NextGen's present exceedingly poor design is nothing but a warmed over 1950's version of ADS-B fueled "Pseudo radar", still essentially using hand carried radar vectors. It will be nothing but the modern day equivalent of having a carbon-boron composite nuclear/solar powered automated buggy whip.