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
What’s wrong with KTorrent? Also KDE has a ton of apps (like 200 of them last time I counted). Many of them are certainly the most popular tool for what they do on Linux. On other operating systems, they’re usually not very popular, tho. Krita is probably the most popular of the bunch and might even be more popular than Plasma. It raises its own funds independently from the rest of KDE, tho. It’s also probably still nowhere near Thunderbird’s popularity.