There are some simple changes which make pretty well understood improvements.

For example rank voting. You rank the candidates when you vote. When counting, you count all the votes for each candidate. You eliminate the candidate with the fewest votes. Everyone who had that candidate as a first choice reallocates their votes to their second choice. Repeat until there's a winner.

Suddenly it's okay to vote for that third party, because through rank voting you can ensure that your third party vote does not end up propping up the candidate you like the least.

That's genius! As it stands, if I were to vote for a 3rd party candidate then I'm essentially throwing away my vote. However, if what you suggest were in place then the risk of a meaningless vote is vastly decreased, essentially eliminated.

Why isn't it used today?

Can't tell if you're being facetious or not.

https://en.m.wikipedia.org/wiki/Ranked_voting_system

Because the people with the power to change it aren't so keen on doing so, I'd guess.

That inevitably supposes that there are a group of individuals with the power to change the voting system.

Does such a group exist?

That is almost exactly how it works in Australia.

You can either rank the vote manually or vote for a party and they get to pick how the vote is ranked between all the candidates.

Running simulations on voting systems is easy.

Also lots of countries have preferential voting systems. Lots of people in AU don't vote for the major parties.

The US system is just crazy.