All the issues you highlight here for subspecies are applicable at the species level or even higher taxa. As I’ve said in other topics:
I would say many more than just “some edge cases” of species do not necessarily have readily-apparent distinguishing characteristics. If you type “complex” in the species search bar on iNat, you will see many, many examples. And that doesn’t even include things like Gallinula chloropus and Gallinula galeata that are so geographically separate that a complex designation is not needed even though they are remarkably similar.
Again, this applies to many higher taxon levels. There are few guidelines determining amount of morphological/genetic/etc. variation needed to be separate taxa at any level.
Again, there are many exceptions to this as well. Subspecies of Ensatina eschscholtzii are very distinct and yet are not geographically separated into “island” populations. While they do have “transitional zones” between subspecies, so do many cases at the species level (e.g., Poecile carolinensis and P. atricapillus). As already stated, the two specific subspecies mentioned by the OP do have visually-distinct mophological differences.
In my opinion, much of the controversy surrounding subspecies is really based more in perceived notions than in actual distinctions between the subspecies level and higher taxon levels. Taxonomy at all levels is messy. It draws hard lines between things that exist on a gradient. While it is necessarily to have such an organizational tool, it should also be coupled with the knowledge that it is not some hard and fast distinction drawn by nature.