Non-voters are idiots but ultimately they will not vote. You can’t lead a donkey to water

I don't understand what you're trying to suggest here. Taking it at face value doesn't make any sense at all - in spite of massively outnumbering third-party voters, the potential impact of non-voters should be dismissed because they are all somehow incapable of being convinced that voting is worth their time? Casting a ballot is a difficult mental hurdle to clear, so it's reasonable to write off anyone who has not yet shown that they're capable of doing so as a hopeless case?

If the argument is that third party voters are throwing their votes away, why should we consider a protest vote to be different in any meaningful way from a protest non-vote?