I mean, sure, if you want
exact mathematical equality to the center of the distribution for 2NdX. But you'll note I said "essentially." I did so for a reason. For the vast majority of things that can crit, this will mean a difference between 1 and 4 points.
Maybe 6-7 if you've stacked several benefits on top of each other (e.g. BM maneuver
and a fancy greatsword, or a magical buff
and (5.0) Divine Smite + Improved Divine Smite).
So you're missing, say, 6 points from an attack doing 24+8+8+5 = 45 (13.3%) or 7 from one doing 24+16+8+5 = 53 (13.2%). The goal of the rule is getting the greatest simplicity possible without
excessive sacrifice in effect (or excessive power-up, but that's not relevant here.) "Maximize the damage roll" is a very, very simple rule--in nearly all cases it's a simple multiplication of two one-digit numbers. "Maximize the damage, and add more damage equal to the number of dice you
would roll" is less simple. I won't call it complex, but the point was that giving up a relatively small proportion of damage (on the order of 12% or less of the 2dX value in most cases...mostly because most weapon dice are d6s or d8s), you get a rule that is dirt-simple, about as simple as it is possible to be. That's a worthwhile trade-off, particularly in an edition that made "streamlining" and simplification part of the ethos of this edition.
I guess your rule would be more beneficial to Rogues, since crits do double sneak attack dice and 5.0 was not especially kind to Rogues. But that could be patched by just making SA scale by 1d6+1, rather than just 1d6, each time it improves--while upping Rogue base damage to boot, which is frankly all to the good, IMO.