Windows Mobile 6.1 frequently utilized Java MIDlet emulators (like Esmertec Jbed). WhatsApp actively supported Java-based phones (Nokia S40) for years. Some CAB files simply packaged a Java emulator pre-configured to run the Java version of WhatsApp.
So, the original WhatsApp is dead on Windows Mobile 6.1. Does that turn your HTC into a paperweight? Not necessarily. If you want to use your HTC as a daily driver or a unique secondary device, consider these alternatives. whatsapp messenger.cab for htc windows mobile 6.1
Use Mobile Office (Word, Excel) for distraction-free writing. Windows Mobile 6