A PCAS that had only a light bulb that came on to warn that an aircraft is within 1 nm would be an improvement over unaided see-and avoid!
Knowing WHEN to look (i.e. STOP whatever else you are doing and lookout in earnest) is valuable.
But, unless every potential threat is co-operative, it won't tell you "when to look". Currently, you have to look all the time. As the percentage of equipage increases, you might eventually find that such a binary detector gave a useful discrimination between times you need to look and times you don't.
Andrews's experiments at the Lincoln Labs that showed an 8-fold increase in visual detection efficacy with traffic alerting were based on TCAS, with azimuthal information on the threats. My experience, albeit a long time ago, with a detector that had no azimuthal information was that the distraction wasn't worth the information.