Trajectory (or volumetric) separation based on RNP works.
If those ranchers at 300 ft AGL in their Super Cub, or gliders occupying a wave block at FL245 to 265 over Colorado or the Sierras, properly define and exchange a suitable RNP defined block (volume) of dynamic airspace for use, ... then even 450kt Vipers on a low-level route or an air refueling track, ...or pipeline patrol UAVs, can safely and reliably miss each other. It isn't rocket science any more, RNP for airspace volumes can work just as we already use it to miss terrain or other aircraft trajectories at places like NZQN or PAJN, ...albeit for GA potentially at 1x10E-4 the cost and complexity of a current jet transport FMS and D/L.
Further, ...yes a Viper radar if used effectively can certainly provide some measure of protection, especially the Block 50s, and yes the new AESA radars will even be better, and yes, it is still typically in the job descriptions of operational squadrons, since before the F86 and F94 to present, to still look out the window, and use the radar when and as needed, especially for MARSA, and they do,...but even when each pilot of a single ship practicing ILSs, or a 4-ship, is doing everything right, the APG-68 has challenges to see small RCS slow movers, particularly with the doppler notch, and if it's adjusted to see these small slow movers at all, it also can display a plethora of other noise (e.g., ground return moving trucks etc). Moreover, we've all seen cases where fighters have already done things like wacking a UAV, or been canopy to gear to a Cherokee crossing the KLUF FAC,...and that doesn't even count a fast mover pilot dealing with rare-normal or non-normal events in a single seat jet. Even in the big birds, with TCAS and multi-crew, it isn't a cake walk, and isn't uncommon for a pilot to have one close call in about every 3000 flying hours. Just in one recent year alone in the US, in air carrier ops, we had over 38,000 unwanted TCAS advisories. So the very concepts of VFR and IFR both now need major reassessment, and evolution, globally. So we all need to cut that Viper driver some slack, at least until the facts are in.