I agree with the original post in general. I think proficiency should trump ability mod pretty much all the time. I've been working on a game based on 5e that uses a 3d6 mechanic with adjusted DCs. The proficiency system stays at +2 to +6, but the ability mods max out at +3. I also allow skills to have +1 specialities; for example, athletics has specialities of climb, swim, and jump. A proficient character, especially with specialties, will beat a non-proficient character most of the time. The bell curve dice mechanic helps a lot as well.
Do you leave expertise alone? This change makes everything succeed less often except saving throw spells vs saves in stronger ability scores, since the DCs and the save bonuses are affected the same amount. Do you change AC formulas? If not, this is a selective buff to anyone wearing medium or heavy armor, since they keep their AC but attack bonuses go down. It also reduces differentiation among skills (assuming characters tend to favor getting proficiency in skills that use their main stats).