Garmin has already added this feature to Garmin Connect. It's right there in the "Time in Zones" tab at the bottom of the activity page.
It may be device dependent as I have a Forerunner 235 and can see the 'Time in Zones' tab in my activities next to Stats, Splits and Segments. The rest of my family (Forerunner 15, 25 and Vivoactive) don't have the 'Time in Zones' tab available.
News flash: Garmin is only putting it on connect modern for activities done on a device that has it available to display on the device itself so that they can sell the latest piece of (admittedly nice hardware) buggy, broken, rushed device with software so bad the chances are it can't reliably display the statistic on the device itself anyway.
My guess is that Garmin executive level has decided the best way to increase revenue is to piss off their current user base.
Why don't you say what you really think?