I think people are starting to realize that now (especially after the pandemic opened people’s eyes to just how many of their neighbors would literally outright kill them, not only by refusing to wear masks but preventing others from wearing one too), but it is debatable how many people were “fully” that way when climate change was first starting to be noticed, and there may also have been more independents. [Edit: in fact, I keep hearing that even in the USA that is known for being so bipolar, only ~11% are on the extreme right and another ~11% on the “left” (for us, setting aside how right-leaning that is on the global scale, but at least it is the side that doesn’t want Trump… which isn’t nothing, see e.g. the funding sent towards Ukraine, even if so late that it may have cost the defense effort, yet still an enormous amount), leaving 3/4-4/5ths that are in-between. So not every conservative is a Republican. Don’t forget: the media lies. Not equally, but indeed on both sides, and both sharing at least one of their myriad of reasons - profit.]
Also there is tremendous value in telling people things - whether they receive it or not is on them, but at least your own conscience is clear, if you shared what you knew. Like if you yelled “Fire!” but someone ignored that and went into the building (but never came out), you may feel terrible but not nearly as bad as you would have if you knew but chose to say nothing, for whatever reason.
Also, the more I think about it, I was being simplistic in my earlier comment: regardless of how pleasant life is or could be, whether humans (or even most large mammals) will continue to exist is a much more pressing concern. So it is not merely an issue of the choice of messaging, it is also about talking about the things that matter most. e.g. Kurzesagt has leaned much more towards the positive side of the message than they used to, but they still focus on the more realistic side of e.g. after our modern technological society collapses what could we expect to look forward to after that - like in a few hundred to thousand years, could we recover [edit: rebuild], especially without access to fossil fuels anymore? Here are two videos both from Kurzesagt: