Esker
Hero
With a D20, though, the numbers are as follows. A has a 35% chance, be has a 50% chance, C has a 65% chance, and D has 80% chance. You're dropping the low guy a bit, and raising the others quite a bit, and that's for a moderate challenge. You aren't just boosting proficiency, but you're boosting success rates. At least in the low and middle ranges. At the high ranges they drop considerably.
That master with his +4 modifier and +6 from proficiency/expertise has only a 1% chance to hit the DC 30 nearly impossible task, instead of the 5% the game would normally give him. During game play when I tell a player that he needs a natural 20 to succeed, he and the other players get excited and gather around to watch, because we've all seen the long shot happen. If I were to change to, "You need a natural 100(2 consecutive 10's) to succeed, they wouldn't get that way, because there's almost no chance of success.
I get the desire to remove some of the swinginess from the game, but it also comes at the expense of some of the fun in my opinion. Most DCs that will be encountered become easier for those with proficiency and expertise, reducing the challenge level of the game, while removing much of the chance of success from the dabbler, who then can't really participate in most challenges that they aren't proficient in or fall into their main stats. And the long shot success roll is pretty much kaput until high level.
It's not going to be for every table; I recognize that. If your players like the chaos inherent in the uniform distribution of a d20, then leave things alone. But for me, if there's a DC 30 "impossible" task, then the fact that the level 5 super-specialist has a negligible chance to succeed gives them more to look forward to. If they take another ASI at level 8, then at level 9 they're able to do the impossible 10% of the time, which is pretty impressive. By level 13, they're up to 21%. Contrast that to the d20 roll, where they're able to do impossible things 20% of the time at level 9 and 30% of the time at level 13. At those levels, doesn't it take some fun out if you're doing supposedly impossible things nearly one out of every three tries? And if you don't expect to get to that high a level, then you still have "nearly impossible" things (DC 27, say) where the super-expert has that 'expanded crit range' chance of succeeding at level 5, and the "super hard" (DC 24) that they need the equivalent of a natural 17 to do. If you just have the impressiveness of the reach tasks increase as characters level, then you give the master a sense of ascending awesomeness.