There is no realistic way to change the two-party system externally, and even if there was, running a national presidential candidate is self-evidently the worst way to do it. There is literally no - zero - chance for an outsider candidate not affiliated with the two major parties to win the presidential race. If the Green Party were serious, they would focus on getting local, state, and national representatives elected in legislative bodies, where they can actually do something. Running a pointless campaign for president to win 1 or 2 percent of the vote is doing nothing. it is influencing no one. No one with a brain could think that it's influencing anyone. When people vote for the Green Party the message isn't "there's 2% of voters we could get if we adopted its policies" it's "there's 2% of voters who likely are never going to vote for us no matter what, because they're willing to throw their vote away for no reason." This relates to the fact that the concept of a "protest vote" is incredibly dumb because whatever the voter meant by making a "protest" vote, their message is unlikely to be received in that fashion.
Contrast that with someone like Bernie. You say he tried to change the system internally and it didn't work, but that isn't right. He is constantly working to influence the Democratic Party internally. He and his compatriots have been successful in moving Democrats left on several issues. When you're in the tent you can use persuasion and influence. When you're outside of the tent you get ignored, and rightly so. You don't have to get elected president to influence the party. Someone like AOC has done 10x more to influence policy and debate on environmental issues, US military use, etc than Jill Stein ever will. And that's just fine with Jill Stein, because her goal clearly isn't to influence or help anyone.