Section 1. The right of citizens of the United States to vote shall not be denied or abridged by the United States or by any State on account of race, color, or previous condition of servitude.
Section 2. The Congress shall have power to enforce this article by appropriate legislation.One of the myths I'm in the middle of debunking elsewhere is that immigrants will be prone to voting themselves a welfare state once they get to America. To make a (very) long story short, the evidence I've gathered does not support this hypothesis particularly well. Along some margins, franchise restriction is occasionally irrelevant.
To make a bolder claim, in a more perfect world, the 15th Amendment would be gratuitous. That is to say that franchise is a valuable possession when and only when political action is important. If the sovereign refrained from extending her authority beyond the scope of enforcing the rule of law, ensuring peace, and upholding justice, then the sub-composition of voting constituents would be close to irrelevant. It is when political elites have retained the authority to dispense special favors that minority voice becomes particularly urgent.
This concatenation is curious. Ex ante, you might imagine that political agents faced a binary choice: shall we trundle towards a limited government that preserves anonymity in the eyes of the law, or shall we take into consideration de facto injustice and attempt to broaden the scope of covenant action to circumclude private transaction? If you choose the former, franchise is mostly irrelevant. If you choose the latter, it is essential. However, the converse is not necessarily true. Just because you have (more) universal franchise does not mean your politics must then march down the boulevard to a civil law society. In 1870, it wasn't clear that the choice was inevitable. Still, if err you must, err on the side of caution.
Is the 15th meta-euvoluntary? I'm unwilling to claim that it had anything to do with the subsequent scope expansion of collective action and the attendant weakening of the political elites' chief duty of providing peace, easy taxes, and a tolerable administration of justice. If there's any claim over the euvoluntarity of this Amendment's meta-EE, it seems mostly retrospective and ex post. It turned out to safeguard the rights of a disadvantaged minority in the presence of weakened institutions. It's kind of lame to say, "well, it could have been worse," but if the shoe fits, you can't make it drink from a silk purse.