Disenfranchisement of any fraction of your population by means of revoking voting rights erodes the glue that makes our democratic state of law stick together.
We acknowledge the laws we have because we periodically get a vote on who gets to uphold, change, and guard the laws.
Removing the right to vote is a breach of social contract.
Those affected then have a just cause to declare the state of law null and void.
The state can respond to this in three ways.
They can acknowledge the breach of social contract and allow the disenfranchised to disregard the law. This is where you are no longer a democracy, but an anarchy.
They can ignore the breach of social contract and still force the law upon the disenfranchised. This is where the state is no longer a democracy, but a dictatorship.
Or they can pretend that revoking voting rights of parts of your population is normal, this is where the state is no longer a democracy, but it is the USA.
(Also, Athens was not a democracy, it was a republic.
And while voting rights for landowners were seen, in those times they became a thing, as progressive and democratic, it was in fact just bureaucraticized feodalism. Or representative oligarchy, call it what you will but not democracy.)