Except for one thing: it's not a natural system, and the candidates do not emerge at random from the gene pool or whatever. They are often or even usually groomed for the role and propped up by powerful parties. Sometimes the parties might be ok with one a short list of possibilities, and sometimes they are deadset on one person, as was the case with Hillary in 2016. Because the system does not randomly generate candidates, therefore your schema fails; they are produced with certain criteria in mind to the extent that they're groomed and supported, which may include electibility, but will also include bolstering and perpetuating the current power structures. It's an exercise in improvement - for them. Unless you're on a board of directors or have lots of stock in their companies, the so-called improvement isn't for you.
Why don't you state outright that there are some mysterious people that selects, grooms and supports the candidates? Because you might have to name names of who these people are?

There were--what?--20 candidates for the Democratic nomination this round. Were they all groomed by these people? If not, then why didn't voters get input on who was the best candidate?
Besides, you are assuming that voters have
no influence on who these mysterious people pick. You forget that, other than perpetuation the current power structure, they have other criteria.
1. Getting their person elected. If their selected candidates always lose, it don't do them a bit a good. They need their person in power to wield that power in their favor. For that, they need the help of voters.
2. Keeping the party popular. Do you believe these people select the Green Party candidates? The Peace and Freedom Party candidates? It would be a waste of time and resources to do so, since such candidates never come close to winning. Similarly, if they select candidates that alienate members of the Democratic or Republican parties, then those parties will become the next Green or Peace and Freedom party, and their back to square one. So it behooves them to keep their selections popular, at least within the party.
There are doubtlessly other criteria, all based on the fact that we, the people, get to vote for who we want. Ultimately any candidate they select has to get the OK from the voters. Which means that we, the voters, do have a say. We have influence whether these people like it or not.
That's not to say that I, as an individual voter, have a huge say in the initial selection of candidates. I'm only one voice. There are millions of other voices, with different values, priorities and criteria. And those with money have a greater voice than those without (since, as the Supreme Court said, money is speech

). So my voice is almost infinitesimal in the overall scheme of things.
But that does not make it insignificant. As long as I vote, and as long as I vote for the candidate that is the best (or even just better) among those who are running, I do have an influence. Me, and everyone who thinks like me. Such a group has influence.
Just look at how much Bernie has changed the Democratic party. Look at the party platform from 2016. You can pick out the lines that Bernie's running for the Presidency added to the platform--lines that those "perpetuating the current power structures" doubtlessly didn't like, and didn't appear in the Republican party platform. Bernie has pushed the Democratic party to the left. So even though Bernie did not win in 2016, and probably won't in 2020, he definitely influenced Hillary and the Democratic candidate this year. And if the Bernie voters keep up the pressure, they will keep those lines in the 2024 platform, the 2028 platform and the 2032 platform, if not adding even more lines.
But for the ones that didn't vote for Hillary, what influence did they have for the past four years? If they don't vote for the Democratic candidate this year, what influence will they have, even if that candidate wins? The Democrats will realize that, since they won't get those Bernie voters even if they add those lines to the platform, they really don't need them. It won't help them get into power. So they don't count.
Why do you think Bernie's ideas and values don't appear in the Republican platform?

So, yeah, votes do count. And so, voting for the lesser evil pushes it toward lesser evil, even if it is only a small push. But it is a push nevertheless. And over time, with a lot of small pushes from a lot of people, it will influence the parties, who they nominate, and what they stand for. But only if the party sees that it will. Not voting for the party means it doesn't matter whether they listen to you or not. They don't get any closer to what they want (to win power). So they might as well just ignore you anyway.
Voting for the lesser evil moves a party in the direction you want. Maybe only infinitesimally, but movement nevertheless.