One need only remember how easy it was to take over IRC channels with a few hundred bots to see the endgame of this rationale… it cannot be patched out, it’s inherent to the internet.
That which would make a vote valid; can (and will) be gamed.
Who said the election needs to take place on the internet?
A paper ballot-style election, while not perfect either, works well enough in practice.
In this setup having users elect the moderator leads to cases where small groups create their special interest group and then some trolls challenge the moderator.
Their may be some oversight on the large sub forum, but not all.
The vast majority of sub forums however are more targeted and smaller to begin with.
For new sites, this meant that the bulk of moderation was done by employees, followed by employee-appointed temporary moderators. This dramatically reduced abuse, but also reduced the explosion of new sub-communities that sites like Reddit thrived on.
I am a big proponent of (direct) democracy in general.
You'd have to weight votes by some kind of participation metric to solve the problem of very little authentication of the voters
Does a subforum offer the same? Once the mod is elected, are you going to sit down with him each day to make sure he is doing the job to your wishes and expectations? I say (ish) in government because it often doesn't even work there, even where people have heavily invested life interests, with a lot (maybe even the vast majority!) of people never getting involved in democracy. A subforum? Who cares?
If there were to be elections, it is unlikely they could be anything other than authoritarianly, with the chosen one becoming the ultimate power.