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.