I don't think that engineers must design product only because of "why not doing it if we can do it". There is a ethical value to what we design and produce. Some technologies are better than others.
I guess you're right, it's just that there are plenty of people who don't think it's unethical to put bluetooth in a toothbrush...? Even if it means putting in a computer which would've been a nice office computer 30 years ago?
Regrettably, electric toothbrushes now often feature displays, Bluetooth, analytics, and the pinnacle of terrible: DRM/forced replacement of brush heads.
These things are so cheap and the containing "smart" product so marked up everyone reaches for the hammer to drive their thumbtack.