There’s no neat and tidy way to blend modes or switch to Open-only play without a significant portion of the player base erupting. Maybe in the next version of Elite if that’s ever going to be a thing, and then only if it’s carefully planned that way from the design stage.
The only way Open-only could be achieved in Elite Dangerous is if the PvP play became insanely engaging, polished, and balanced, such that all participants in a PvP encounter, whether they be geared for combat or not, will almost always have an interesting experience, with more and better choices in any scenario. If that were done so well that almost no one continued to choose to play in Solo or PG modes, those modes could be switched off, leaving only Open play.
But that is never going to happen, so it is what it is, and players must needs compromise and use the existing modes that work best for them.