1. Telemetry is unethical if the users didn't provide informed consent (opt-in) for it. It's not just a theoretical point - anyone who's worked in tech sector for a while should know most companies cannot be trusted to behave ethically (especially if they took VC funding).

2. There's a certain dysfunction/antipattern that's popular in tech sector, called being a "data-driven company". It's the practice of making decisions through divination from data collected through extensive telemetry, to the exclusion of other knowledge sources (like e.g. actually talking to your users, hallway testing, or thinking things through). This leads to software being optimized in questionable directions - so in a sense, you could say that adding telemetry implies an increased chance the software will become worse over time.