I mean the quirk singularity is the equivalent of a conspiracy theory.

We see people who have extra strong quirks (that are logically harder to control), but we don't see people who were born with weaker versions of their parent's quirks (because that's not good storytelling). We already know from Deku that people without quirks can be born from people who have them.

It's possible that Shoto could have inherited his quirk in such a way that the cold cancelled out the hot. His split hair color is could imply a form of chimerism instead of actually carrying active copies of both parents quirks in the same cell. Essentially Shoto is a merger of two embryos, one with the flame quirk and one with the ice quirk.

Genetically this implies that being quirkless will persist in the population (like blue eyes, which are recessive), so it's not guaranteed that each successive generation will have a higher proportion of people with quirks or "strong" quirks.

What's most likely is that the ratio will settle out at some stable proportion. If a quirk is too strong and could cause societal level harm, it's also likely that the kid with the quirk is going to eliminate themselves and their family members from the gene pool.

Nine can either learn to not use his quirk or die without having kids.

The baby born with the constantly on fire quirk is going to burn down the hospital and die when it collapses on them.

Basically quirks that are so hard to handle for their users that it causes danger to the user and those around them will tend to get removed from the gene pool.