I've proposed a theory that Tolkien never saw or commented on
Meaning this is wholly my idea, but why not have "many" Balrogs exist before the fall of Utumno, leaving seven, these seven to further dwindle at the Fall of Gondolin, War of Wrath, and one at the much later Moria encounter.
It seems a compromise of sorts to me, although again I've no idea if Tolkien ever contemplated this himself, or if he was going to note an exact number of Balrogs in the story itself. For example, revising "host" to "his" doesn't exactly reveal much in the way of numbers, even if externally we see that it represents a drastic reduction as far as the older references are concerned.
In my opinion, Christopher Tolkien's revisions to
Quenta Silmarillion follow Tolkien's lead here, generally speaking, where he edited out lingering textual references to large numbers of Balrogs . . . as, despite the note just raised, for whatever reason, JRRT did not himself edit out every reference to large numbers.
Again that story doesn't end there even, but for now I'm goin to get ice cream.