It was just a throwaway comment I made because Ash has already released his only Pseudo-Legendary, his only Mega and his only UB. Meltan is similar, a potentially super-powerful and special Pokémon.
Except in two of those cases it really wasn't permanent considering Goodra situation is more of that of a pokemon that can be called upon, and it doesn't take a rocket scientist to see that how they worded Greninja's leave was more indicating that it was a temporary leave rather then a permeant one.
Also Poipole can't really be used to say that Meltan will leave because when Poipole first met Ash they hinted at it leaving by having Lusamine tell Ash that he could watch over it until they find it's ultra wormhole, where as there was nothing that would indicate that Meltan would eventually leave when it joined Ash.
Plus your argument could literally be said about almost any pokemon. At this point saying any of the current pokemon on the main cast are definitely getting release when there is nothing in the series that indicates them getting release is jumping to conclusions.