Let's consider the consequences of that line with respect to electoral math. If we consider only serious criminals, e.g. murderers, they constitute a negligible proportion of the population and with high probability the number of election outcomes changed by allowing them to vote or not would be none.

By contrast, if you lump in people convicted of things like drug possession, that is enough people to change the outcome of some elections. And in general it's a strong heuristic that if huge numbers of people are committing a particular crime, it's a result of flaws in the law or society rather than flaws in huge numbers of different people.

So the only time disenfranchising felons matters to the outcome is when you get the line wrong, implying that it shouldn't be done because it shouldn't affect the outcome unless it's being done improperly.

The big issue are perverse incentives here. If felony sentence means no vote, the best thing you can do is to criminalize demographics you dont like as much as possible.

That way you can have pleasure of mistreating them and also prevent them from voting.

Unfortunately you're also engaging in an appeal to universal virtue.

It's weird because your argument doesn't seem to disagree with the notion that people should stay enfranchised, other than you saying specifically people should be disenfranchised for breaking a law. But you're now discussing lines so I guess you mean, literally any crime means no more voting.

A good democracy, and by that I mean useful for humans, isn't good by trying to be perfectly virtuous, it's good because it has recursive mechanisms to maintain its usefulness to humans. The primary mechanism is voting. For that reason I personally believe nothing should be allowed to remove the ability to use that primary mechanism, since the obvious outcome is a fascist is elected, and begins seeking means to strip the right to vote from his opponents, ensuring his perpetual rule. Modern example: I have a little antifa flag on my backpack, and therefore am now considered a terrorist in the USA, and can be arrested and have my right to vote stripped (other democratic mechanisms might prevent this, for now).

What crime would I have committed? Declaring an ideology a terrorist group is nonsensical but possible. Me suddenly being a terrorist crossed that line for you though.

So does speeding. So does operating your motor vehicle without checking your brake lights and turning indicators, every time. So does riding on a horse backwards in a specific town in Texas (don't forget local jurisdictions have their own laws, often insane!)