Yeah agree-so looking at ways to balance post-rolling so we get the fun of rolling and balance with post-hoc feats
Bear in mind that some feats provide ability increases which could cancel out the penalty.
A fighter that's just under the cut-off point by one or two ability points could pick Athlete and get +1 Str, +1 Dex and be par or even slightly better than a point-buy character's ability array and get the Athlete's standing up from prone ability on top of that.
'Course that's an unlikely edge case, but it's worth considering.
It's more likely that you'd have a character with a good number in their primary stat (since the Player would likely have their best stat in their class's primary ability, either by assignment or random luck depending on what ability assigning rule the campaign used).
The Fifth Edition RAW says a Feat has a value equivalent to an Ability Score Improvement from class advancement, so if you're adding this Homebrew I'd think you'll want at least a 2-point shortfall on the ability array before it kicks in, equivalent to a character with a Point Value of 23 or less (The standard 27 points of point buy less 4 for the cost of increasing two decent ability scores by +1). Maybe a bit less, since Feats have an upper limit of 20 on an ability score like standard Advancement, not the 15 of standard Improvement, plus there's the possibility that your random roll can already have 16+ ability scores.
Let's consider some extreme cases.
If a randomly rolled Ability Score of 18 is worth 15 as per the original proposal and the trigger point is a point value of 23, that leaves 8 points. A character with the array 8, 8, 8, 9, 14, 18 or 8, 8, 12, 12, 18 would have that cost. If they get a post-hoc bonus feat, they can boost one or two stats by +1 with the right picks to make their prime stat to 19.
A more average character with randomly rolled scores within the point buy range of 8 to 15 can easily have one or more 15s and a point value of 23. The most extreme case would be an array of 15, 15, 13, 8, 8, 8. If such a character got a pro bono feat, those two 15s could become 16s for a point higher ability modifier.
'Course it's unlikely to randomly roll some of these stat arrays, but the unlikely can happen. Plus random generation has the problem of fudging rolls that simply isn't an issue with point buy.
Plus the racial ability adjustments can make a big difference. A +2 in the primary stat would make that roll of 15 an 18 once it's boosted by feat and race, while a roll of 17 (which has a 4.1666% of happening with best-3-of-4d6) would go straight to the advancement max of 20.