You can mix them in the same house but why? There's a common misconception that conventional systems can't be installed in small spaces. FYI, we have conventional oval ducts that easily fit into a wall cavity. We have flexible ducts that can be PROPERLY installed in tight spaces. And closets on the first floor are not generally a consideration for bringing ducts from the attic down. It's closets ont he 2nd floor that would be used to access rooms on the first floor. Sometimes it's necessary to box in a duct in the corner of a room to hide a duct that can't find a closet but 95% of the time a conventionial system can be installed if the owner has the desire to have it done. We install both HV and conventional but the HV is more money and IMO, a poorer result with less efficiency. But there is not reason you couldn't install both types of systems in a home, just not both types on the same system.
If YOU want change, YOU have to first change.
If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!