Stage Manager at least, requires an Apple silicon M1 or M2 CPU. The much older A13 CPU used by the iPad 9 does not have the speed or resources for Stage Manager since it needs to keep many apps open and available at the same time, and has to switch them seamlessly. The older CPUs would cause lag and an unpleasant experience if they were to support it.
It's a fact of life cheaper products have less features. You want more features, you need to pay for them. There's a reason the iPad 9 is almost $300 cheaper than the cheapest iPad Air that can support Stage Manager.
What you are proposing is to make the more expensive iPads redundant and pointless when you can get all their features in a cheaper older version? What would then compel anyone to buy the more expensive ones?
The more expensive iPads get more features. That is a fact of life with things. Does a 25,000 car get the same features and niceties as a 60,000 car?