I agree this is reasonable from the individual publisher standpoint. I once received feedback from a reviewer that I was "searching for the minimum publishable unit", and in some sense the reviewer was right -- as soon as I thought the result could be published I started working towards the publication. A publisher can reasonably resist these kinds of papers, as you're pointing out.
I think the impact to scholarship in general is less clear. Do you immediately publish once you get a "big enough" result, so that others can build off of it? Or does this needlessly clutter the field with publications? There's probably some optimal balance, but I don't think the right balance is immediately clear.
Playing devils advocate, there isn’t a consensus on what is incremental vs what is derivative. In theory, the latter may not warrant publication because anyone familiar with the state-of-the-art could connect the dots without reading about it in a publication.
Ouch. That would hurt to hear. It's like they're effectively saying, "yeah, obviously you came up with something more significant than this, which you're holding back. No one would be so incapable that this was as far as they could take the result!"
Thankfully the reviewer feedback was of such low quality in general that it had little impact on my feelings, haha. I think that’s unfortunately common. My advisor told me “leave some obvious but unimportant mistakes, so they have something to criticize, they can feel good, and move on”. I honestly think that was good advice.
I think the impact to scholarship in general is less clear. Do you immediately publish once you get a "big enough" result, so that others can build off of it? Or does this needlessly clutter the field with publications? There's probably some optimal balance, but I don't think the right balance is immediately clear.