This was actually one of the proposed changes that was being tossed around a couple months back, but it never got any traction because we were admittedly in a 'complain about the massive feedback loop that all of the current mechanics feed into each other' phase.
That said, this is a pretty elegant solution, because the current heavy emphasis on advantage/disadvantage leaves spells that grant advantage/disadvantage and those that target saves in the dust, as you've already said. Raising proficiency bonus would be a far more balanced approach than what we currently have, AND it would give players a slightly better sense of progression from leveling up (as advantage/disadvantage is a static mechanic, while proficiency improves as you gain levels).
