It seems to me there's a big difference between an explanation of an expected/obsevered mismatch which says there must be another massive object involved - and here's where to point your telescopes to look for it, and one which says well there must be some other mass of some form somewhere.
For a pretty long time, for both those planets, they didn't know where to point their telescopes. This was also true for the outer planets, and Pluto too. A similar sort of thing, to a different/lesser extent, used to be the case with gravitational waves not having been directly observed despite other evidence. There were similar problems with the early discussions of atomic nuclei, electrons, neutrinos, neutrons, where all kinds of things were posited based on their effects only. I know it's quite a bit more difficult, and less direct, with dark matter, but still.
(Really, I was objecting to the use of the word placeholder, because I found it unfair.)
