As I see it, there are two different tracks one can follow when discussing issues such as the UBC. One track is the theoretical or philosophical where you can talk about things like the Founding Fathers' original intent. The other track is the practical... the pragmatic. On this second track, your discussions are constrained by what is possible or probable given the entirety of current social, legal, and political conditions. We become frustrated - often angry - when we're on track one and we try to carry on a conversation with someone on track two. In this situation, our conversation is not supported by a common frame of reference and will seldom rise above the level of friendly bickering.
In the case of the Manchin-Toomey amendment, a pragmatist would recognize that NICS isn't going anywhere. There may be better systems out there but NICS is here to stay for the foreseeable future - too much has been invested in the legal, technical, and procedural infrastructure for NICS to be replaced or abandoned. But NICS can be improved. The States can upload more criminal and mental health records. The restraint against a centralized registry and related penalites can be codified more forcefully. The interstate transport of firearms and ammunition can be made more traveler-friendly. And the process for rights restoration can be strengthened and simplified.
Is the M/T amendment the right legislative vehicle to make these improvements? We will each decide that for ourselves. Politicians love to wax philosophical when standing before the cameras. Their careers and their votes, however, are very pragmatic things.