Are you aware this very likely results in a significantly higher average than if the DM just stated "everybody starts with the default array, full stop."
It really isn't significantly higher... it is marginal really. The chances of wanting to replace the rolled scores with the 5E standard array is about 41%.
Rolling but allowing replacing with standard array helps in the long run by about 2 points higher, and has no impact overall on the the ability modifiers compared to roll 4d6, drop lowest, at the start.
Expected scores for rolling 4d6, drop lowest:
16, 14, 13, 12, 10, 9
Expected scores for rolling 4d6, replace bad array with standard array (15, 14, 13, 12, 10, 8):
16, 15, 13, 12, 11, 9
So, the 14 becomes a 15 and the 10 becomes an 11. In both cases, the ability modifiers remain the same (+2 and +0).
If you want to offer an "inferior array",
14, 13, 12, 11, 9, 7 is the expected value for the arrays which would likely be replaced after rolling and bring the expected value of the entire system on par with just rolling 4d6, drop lowest.
The OP's suggested "inferior array" (15, 13, 12, 10, 8, 6) is just a bit over-punishing in terms of ability scores, but interestingly enough the ability modifiers with both "inferior arrays" would be the same.
Anyway, it's been fun.
