Issue is that she's turning off the very people that needs to be educated.
Young folks would probably resonate most with her but they're already mostly already educated about global warming in the first place with science drumming on about it for very long already.
I think this is a fundamental misunderstanding of how protests and movements works. Very often the goal is not to convince people, it's to mobilise the people who in varying grades are already on your side.
I'll do the go-to comparison I always make, the American Civil Rights Movement. Martin Luther King jr, and the movement generally, didn't convince people that black people should be given more rights. In fact, as the movement went on it became more and more unpopular, and the same rethoric as people are directing at BLM now was widrespread (violent, riots, communist, etc). What the movement did was to get people together, mobilising people who were for the goals, and use that to create pressure and political capital.
A 40-something guy who doesn't care about climate change doesn't matter. He wasn't going to do anything anyway, so how he reacts to Greta is irrelevant. Likewise, those who deny climate change are also irrelevant. She's telling people to listen to the scientists, the scientists have spoken for decades and the denialists have kept on denying. How they react to her is irrelevant.
What a movement like this is doing is taking people who already know that climate change is real and a threat, primarily young people, and make sure that the topic is out there and that people actually care about it and think about it actively. If you work with kids, or young people, then it's pretty amazing how much they talk about climate change. How high up their list it is when talking (future) voting and politics. Of course that's not all down to Greta, that'd be absurd, but she is having an impact on a lot of people's lives and attititudes.
One of the groups MLK wrote and spoke most scathingly about was the W
hite Moderate. Not the KKK guys, but the moderate white people who, sure, in their hearts of hearts believed that black people should be given more rights, but didn't actually do anything. Not right now, not yet, not this way.
Eventually. These were people who in principle were on the side of black people, but in practice they were not. The goal of a lot of movements is to both energize the people already in your camp and to force the moderates to act on their beliefs. A lot of White Moderates would vote for equal rights, but they won't do shit to make a vote happen. So you force the issue and make them vote. Some conservatives turned even more racist in the process? feck them, this wasn't about changing hearts.
The road from what young people care about to what politicians care about is long, of course, which is a big drawback when we don't have that much time to act.