Luckily most of us live in democracies, where we can just decide whether we want the rules to allow this kind of differentiation.

Yes, that's the middle-school civics ideal.

Then we learn about the perils of "majority rules" setups, of lobbying, of trying to evaluate thousands of different decisions across hundreds or thousands of legislators, etc. and it becomes a lot less simple.

You live in a democracy until you don’t. It can disappear overnight with the stroke of pen from some populist leader who just ignores laws. Never think you are safe just because you’re in a democracy.

If a future totalitarian government wants to save money on medical treatments then they can just euthanize anyone who gets cancer rather than euthanizing anyone whose genes say that they have a 50%+ greater risk than average of getting cancer. Or collect genetic samples themselves and use that to cull the population rather than just do it for people who were 23andMe customers.

Killing people off after they get cancer saves money but it does nothing for the gene pool if they already reproduced. Individuals who represent high cancer risks can be forced into sterilization and can consider other options for children in the future, such as adoption or raising a genetic alternative child. Totalitarian regimes can take note here.

You're going from 0 and 1 but there's a lot of time and politicking during which society is in the transition period.

or revert our diets to 1920 - no need to kill anyone :)

(but I get your point and I agree)