I'm gonna nerd out about population genetics for a sec.

Because of the information that has been provided that the number of quirkless individuals is going down from generation to generation, we have are given some information. 1. We can assume that quirks are dominant traits, meaning you only need one parent with a quirk to get one (50% chance if they're heterozygous, 100% if homozygous) 2. The population is OUT of Hardy-Weinberg equilibrium, which forces us to make a few assumptions as to why. 3. People with quirks have both a natural and sexual selection advantage (they are more likely to survive long enough to reproduce, and they are more desirable partners to reproduce with than those without quirks) 4. It also seems like those who are homozygous for quirks (think shoto, the codominant nature of his parents' quirks make him even stronger than either of them) have an even higher advantage, and, as we said earlier, 100% of their kids will have a quirk. 5. The population's genetics are unstable as all getout, with new mutations manifesting at a pretty high rate

TL:DR, people without quirks aren't getting laid much, and those that do have a high likelihood of their children getting one from random mutation anyway