With Apple still having only 3gb of ram in its new iPad 9gen, which I believe is gonna be the key factor in not being able to recieve new iPadOS updates, I think the 7gen is gonna be supported for quite a few more years.
However, with Apple also starting to support older iPadOS version as seen with ipadOS14, it might be in order to start seperating which devices get updates while still providing Long usablitity for all devices. Hence, creating a gap between pro and budget devices.