Expansion of the franchise seems to be a law of history. There will always be a faction within the ruling class which can benefit in the short run from expanding the franchise, though it doesn't usually benefit them in the long run.
Expansion of the franchise is a ratchet. Once you give people the vote, they will not vote for a candidate who proposes to take it away from them. Like the immigration ratchet, if you propose to take the vote away from 25% of the electorate, you need a 2/3 majority of the remainder of the electorate in order to have an overall majority.
Could there be a situation where a majority of the electorate unites to disenfranchise a minority?
Certainly. If the electorate is divided racially, a majority race can unite to disenfranchise the minority. Often, the minority does not lose the vote, but has various sanctions imposed. In South Africa, there are quotas against whites in business and academia.
In business, there are laws to prevent majority shareholders disenfranchising minority shareholders. Dividends must be paid equally. Majority shareholders sometimes attempt to get around this by employing themselves at a high salary or awarding contracts to themselves, diverting profits into wages and "consulting fees".
Redistributive taxation is the poor majority uniting to financially disenfranchise the rich minority. You can't have negative inequality, therefore unless there is perfect equality, there can always be a coalition of the poorest 51% for taking the wealth of the rich. A coalition of the richest 51% is less likely, because the people in the middle get a higher payoff taking money from the rich than they do taking money from the poor.
Redistributed money will not be split equally between the poor; it will be split so the poor are equal. Otherwise it creates incentives for redistribution in the other direction. Consider a population of three people, A, B and C, with wealth as follows: A 1, B 3, C 4.
If A and B unite to take C's money and split it 2 each, the new distribution will be A 3, B 5, C 0. Now A and C have an incentive to unite to take B's wealth.
Instead, A and B take C's money and split it so they are equal. The new distribution is A 4, B 4, C 0. This is stable. Therefore it is possible to maintain inequality in a democracy as long as the upper class is equal and a majority. This is unlikely.