Often times people have resolved all the rational arguments to act on a decision but lack on an emotional excuse to figuratively pull the trigger. I’d bet on someone high up had already made up their mind and you not using WhatsApp was the perfect excuse to just have the whole team finally migrate.
Lesson time. In security strategy we have the risk equation. The calculated risk of something is the magnitude of the harm it could potentially do, times the probability of it actually happening, all divided by any prevention measures you have or can take. Nothing is perfectly or inherently safe or unsafe, you always have to calculate the risks taking into account all the factors, and balance risk against operative costs. There’s a lot of economic value in a low risk system that doesn’t require much intervention or maintenance.