Just for starters, what if you have two people in the house who want to do a Fitness+ workout together? Too bad: even if they both pay for it, one gets the nice tracking and HUD and the other gets squat. This is an obvious and trivial feature, and it’s nowhere to be found. I could maybe see it getting cut for the launch checklist if people were behind schedule, but Fitness+ plus is more than five years old now, there is no excuse.
It’s total abandonware from a company trying to do the absolute minimum to get your recurring subscription.
They do support syncing up the workouts of people who're each using their own device: https://support.apple.com/en-us/101979
What? Abandonware? They are constantly posting new workouts that are thought out and well produced. The Fitness+ app is very well maintained. It works great. It has cool features. Honestly I don't know what you are talking about, here.
I've worked out together with one to three other people several times. No one cared that their heart rate wasn't shown on the screen. It's really not an important feature and a very niche use case.