Total everything up and if you are net payer you get to vote. If not no vote for you.
Even skipping the moral argument, there's a game theory trap here: let's say you run a government and your political enemies are on the poor side and fall into the "no vote" trap more often than your supporters. Obviously the solution to maximizing your share of the vote is to make them even poorer.
Then again I suppose it would be fine too if voters decided to socialise absolutely everything including bodies of other voters. After all that would be democracy. And they chose that no protections are needed and everything is common now.
So why does that principle change when some people are poor or when the tax structure is progressive?
What happens when the federal government passes new tax laws that prevent certain genders, racial groups, etc from having to pay income tax? Are they permanently disenfranchised? This would be a system that I think would rapidly move social progress backwards and concentrate power in the hands of a few groups.
And USA already disallows voting based on age. And somehow lot of people find it acceptable. Surely voting should be right from birth?
Also, surely you don't think a four year old should weigh in on who gets to represent them in Congress? We disallow voting on a minimum age, because children are still developing in all sorts of ways, there is no maximum age that takes away your right to vote when you reach it. You can't equate a minimum age to disenfranchisement.
And really why should not 4 year old get to decide? How come you think it is right to discriminate against them based on their age? As you said there is no limit on those with limited mental faculties due to old age. So why have one for young age? Isn't it clearly an unacceptable form disenfranchisement? Or are you saying some forms of that are absolutely acceptable and even justified?