Is simplified subsumption really necessary?

[Chris Smith]

This might be in the "ship has sailed" territory, but I'd like to bring it
up anyway.
https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0287-simplify-subsumption.rst
says:

Suppose GHC lacked all four features, and someone proposed adding them.
> That proposal would never leave the launchpad.
>

Let's test that hypothesis.

I've been spending increasing amounts of time fighting against simplified
subsumption while porting Haskell code to GHC 9.0.  It's not that any
specific instance of this problem is hard to fix; rather, it's that it
completely screws up my intuition about what should be valid Haskell.  It
doesn't help that HLS still requires 8.10.4, so I usually don't find out
I've broken my libraries for GHC 9.0 until continuous integration kicks
in.  At this point, it's become fairly routine that my code that works fine
with 8.10.4 is broken with 9.0, and this makes me sad.

Understandably, eta expansion reducing the strictness of terms is bad.  But
wouldn't it be possible to choose a desugaring with seq that doesn't do so?
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