The reason why it matters less than people intuitively think is precisely that it matters a lot less when you're in full control of both the operations and the types anyhow, and that's actually the most common case. Generally you are "composing" in library code, that is, just using it, not extending the library itself.

When you are extending, you actually want to choose the correct thing depending on what you need. Going the wrong direction is painful in both directions.

Personally I think one of the reasons sum types are greeted with such "oh my gosh where have you been all my life" reactions is precisely that we had type extension as our only option for so long. If we had had only sum types, and type extension was given to us for the first time in obscure languages 20 years ago and they only really started getting popular in the last 5 or so, I think they'd be considered in much the same way. Just as in a world with only screwdrivers, the invention of the hammer would be hailed as a revolution... and in a world with only hammers, the invention of the screwdriver would be hailed as a revolution. But in both cases the real mystery is how the hypothetical world got that far in the first place.

Not that they aren't useful; consider what it means that I'm analogizing them to something like a hammer and a screwdriver, not an oil filter remover or something. It is weird that we were missing one of them for as long as we were in the mainstream.