Restoring "the footnotes" blindly makes no sense as one of them is completely unaffected by these changes (the AC-related one). Why would you reinstate a change that isn't a double-change?
Why don't you think it's a double change? The base went down (except for the highest level of Hard) *and* the footnote went away. That's two changes.
Don't think of it as a DEX roll, think of it as a DEX attack, where this table sets the base Defense. Those defenses are now lower by a significant degree, especially if it's a Weapon attack against AC, which is now advised by this page (thankfully, the later monster design advice is unchanged) to be the same despite Weapon attacks being inherently 2-3 points easier.
Per the revised table, the proposed improvised AC for Hard targets at 1-3 is only 15. Yet we know from the MM that 17 and 18 is where the actual numbers often lie. That's what putting back the +2 footnote would restore.
Per the revised table, the proposed improvised DC for a Hard skill check at levels 1-3 is 15. Yet a character with no mods at all has a 30% chance of succeeding at that target, and anyone trained at all is over 50%. That's not anything I'd call 'Hard'. Restoring the +5 footnote would make the improvised Hard DC 20, which is much more in line with its purpose. Now, as we go up the table, Hard will move out of the range of the no-bonuses-at-all character, but I have no real problem with that, because it's the definition of "Hard". Especially since the skill challenge rules no longer over-use Hard checks.
Am I 100% sure that restoring the footnotes is a perfect fix? No. But I'm rather convinced that adjusting the table *and* removing the footnotes has resulted in Defenses/DCs that are too low in easily observed scenarios.