These are Matter/Wi-fi devices, not Matter/Thread devices, so I don’t think that part applies?
No, this is a property of Thread and not limited to Matter devices. For example you can mix Thread/HomeKit devices with Thread/Matter devices in the same Thread mesh.
Only wired devices act as range extenders though, like smart plugs or outlets.
Currently Matter devices are indeed more expensive but they have the potential of being even cheaper. Matter also supports Wifi and Ethernet, not only Thread.
With Matter, device manufacturers can focus entirely on the hardware/firmware and don‘t have to worry about client software like custom apps. They only have to implement a single protocol to support all major ecosystems natively.
In theory this should also lower the bar for entering the market for small companies.