Or 14 dex versus 12. That would matter for ranged weapons and initiative and a boat load of skills. I mean, honestly, if a +1 is substantial, as you proved it was, then a +1 somewhere else, especially something associated with more of the character, is just as substantial.
False.
First of all, this varies between campaigns. In certain campaigns where combat tends to have a lot of enemies, instead of one BBEG or just a couple powerful enemies, initiative will be more important to have a bonus in than in the campaign with less enemies. This is because, on average, the less enemies there are the more likely you are going to be able to go before the majority of the enemies. It's easier for a paladin to wait 3 turns in order to take the boss out with a nova attack than it is for the paladin to wait through 10 turns as goblins wack them with sticks. Even though the goblins are less deadly than the more powerful bosses, the paladin will care about initiative more the more enemies there are. A player simply would rather wait through 3 turns of dramatic, BBEG-style combat than wait for goblins 1-11 to get over with their turn already.
My point here is, the importance of the different ability scores will differ between campaigns. The paladin with a +0 dex may appreciate the +1 dex in the campaign with a horde of goblins every combat, but the paladin with 2-3 BBEGs each combat will be more than willing to trade a +1 in initiative for an additional +1 to their attack/damage rolls or spell save DC.
Second of all, not all ability scores are created equal. For your example, you chose the most useful ability score in all of 5e, Dexterity. Dexterity and Charisma are objectively more useful in the game than Strength and Intelligence. No one will complain about having a higher Constitution, as that will give them more hit points, and most people won't complain about having a higher Wisdom, because then they get a higher passive Perception score and higher saves against charm/frighten effects. However, a Barbarian will (more often than not) not care about having a bonus to Intelligence or even care about it. A Rogue will not want or care about having a higher Strength score, as more of their features are Dexterity based than Strength based.
Dexterity is an ability score no one will complain about having more of. However, there are other ability scores that are not as good or as useful as Dexterity, so using it for your example is a bit of a fallacious argument.
Third of all, a +1/+2 to your main ability score is hell of a lot more useful than a +1 to an ability score that you don't care about. A paladin does not care about their Intelligence score (the majority of the time), but do care about their Strength (or sometimes Dexterity), Constitution, and Charisma scores. A Gnome/Mark of Making/(any race that gets an INT bonus) Paladin would way rather have a +1 to their Strength/Dexterity, Constitution, or Charisma modifier than their Intelligence modifier because they benefit way less from an INT boost than they did to one of those ability scores.
My point here is that not all ability scores are created equal between the classes themselves. An Inquisitor Rogue would way rather have a +1 to their Wisdom score than their Strength score, and a Moon Druid couldn't care less about their Charisma score. A Paladin that uses a halberd, plate armor, Great Weapon Master, and Polearm Master does not want or need that bonus to Dexterity. They typically don't care about initiative (since they don't have any mechanical advantages to going first in initiative), they don't care about ranged weapons because they're a paladin, and they don't base their AC off of Dexterity anyway.
Conclusion: Your argument is flawed. It is not equally beneficial for a character to get a +1 to any ability score as it would be to have a +1 to one of their main ability scores.