Considering I often come to things like Show HN/Ask HN to see what exists in the world, yes. I say it is not only appropriate but within the spirit of fostering good conversation and interest in the community. (Obviously only as such competition remains civil.)

Absolutely, especially in such a crowded mature space it makes total sense to discuss the full market. As others have said, I like to come to the comments to learn not just about the product at hand but of what else is out there, show hn isn’t some exclusive show room for the product in the title.

Why not?

Also, it’s a Show HN.

Seems reasonable to share alternatives.