A better cheaper solution assuring airspace access is possible
Yes the F-16 radar could theoretically have spotted the C150 if the pilot had the time to share the complexity of single-seat jet high speed flying duties with added radar operation too, as well as at the same time still assuring "see and be seen", to the extent possible, ... all when he thought ATS was comprehensively providing separation service.
But the long term backup separation CAS solution is ADS-B related automated and nearly instantaneous 3D or 4D RNP based trajectory exchange among vehicles, WITHOUT ANY NEED for any ATS intervention (and NOT the severely flawed poorly designed FAA 91.227 version of ADS-B, which is still both fatally flawed as well as overdesigned, in many respects). Also noteworthy, is the fact that TCAS as presently designed, while it is an excellent tool as a start, particularly for expensive high speed aircraft, is presently far too expensive, too heavy, and is even too limited in capability, and spectrum inefficient, for either the needs of F16s, LSAs, UAVs, or even B777s, and A350s for the long term.
So the type of ADS-B needed for that C150, and everybody else in low end GA too, was a small, light, low cost unit (maybe even portable) of under $500, and NOT needing FAA's absurd overspecified, excessively costly, inappropriate 91.227 driven NIC and NAC, that can be placed on every vehicle in mixed airspace. That way any vehicle from LSAs to parachutists, to UAVs,... could have been visible to higher speed traffic. With a simple device like that on every manned air vehicle, it would have allowed for simple display of cooperative traffic and trajectory assessment on the F16, WITHOUT ANY need for ADS-R (hence the needed move to EFR, and no longer dependence on either VFR or IFR). Fire control radars typically have a difficult time spotting and reliably tracking some very small low speed GA aircraft and distinguishing them from birds, let alone seeing very tiny UAVs. So only then, with a better low cost ADS-B solution, and cooperative automated conflict probe and resolution by ATS (based on RNP trajectories or RNP volumes for certain kinds of airspace operations) and data links for their exchange, with use in most mixed airspace, will there be any reasonable probability that we can economically keep F-16s from running down C150s or gliders, or B777s or A350s from fatally ingesting errant "link lost" tiny wingspan UAVs into their GE90-115Bs or Trent 7000s, such as while flying into a sunset landing on 25L at KLAX, or popping out of a puffy cumulus or cloud deck, as fatally happened to AL853 descending into KIND in September of 1969.