The post where you indicated things 'feel' correct? It wouldn't feel correct at 5% + or - ?
Considering I mentioned a range of +6 to +8, I'm sure it would.
+4 or +10, maybe not at those levels.
+2 or +12, I would say not for your main focus. You would either consistently fail or eclipse your team.
If you are looking for a pronouncement of inescapable fact handed down from on high that +7 is the One True Bonus at level 8, I don't have that. What I do have is the observation of how the bonuses accrue. In 5e it seems to be Attribute + Proficiency which proceed in a predictable manner. At a particular range of the game which IIRC has been chosen as most enjoyable, you have roughly 70% chance of success in common tasks and 40% with the hard tasks. Given how crappy people are at estimating probabilities and that we remember failures more than successes, those percentages track well with how results feel.
D&D is a social game where how people feel about results and their character's capabilities are important. The sooner you reach the "sweet spot" probabilistically the more content people will be with their characters and the easier adventures are to design by the DM. The probabilities will change slowly from that point unless a power or item is used, and then just for that encounter. If you take the ASI at 4th level and a feat at 8th, whatever your bonus is at 1st level will increase by a total of +2 at 5th and then only by +1 more by the time you reach 13th level. That is a long time with little change to the base bonus.
So by starting with a high score in the prime requisite you may reach that spot sooner. Perhaps, then, things may seem easier at first, but when in that range from 5th to 12th level what matters more are the powers the character has more than the actual bonus. I do assume that the developers made some decisions as to what monsters and challenges the characters would face during those levels. I also believe that they expect a certain attribute range as a foundation for their assumptions as to character capabilities. After doing some pretty simple math, I conclude that the assumption was an 18 in the prime requisite, perhaps 20 if attributes were rolled, at 4th level after the ASI.
I have a feeling that this is what you want to hear: Is this mandatory for an enjoyable game? No. Is it required? No. Can someone design their character some other way? Sure. But, I think this is how the developers designed the game. And, thus, influences the choices that the players make and expect.