I can’t find KDE’s financial report, but in a video I watched it was claimed that Thunderbird collected more donations than KDE. It seems quite hard to believe, but in 2022 Thunderbird collected more than 6,4 million dollars.
KDE is an entire desktop environment, with a bunch of applications and even partnerships that have yielded a KDE laptop. Should Thunderbird have been able to collect more money than KDE itself, there might be something that KDE can learn from Thunderbird.
Edit: Added the link to the video that I watched
They have KDE connect and some other tools but I’m not aware of how popular they are even for Linux users. A lot of it is duplication of other already popular tools available. And their bittorrent client sucks in my humble opinion.
I would expect for most users KDE is just another window manager for Linux. Secondly it’s not even the most popular. So I would expect its user numbers to be much smaller.
Krita has been downloaded in the millions. Kdenlive is getting there, as is KDE Connect since they now support Windows and macOS. GCompris is used by 10s of millions of kids in schools and homes all over the world.