When the use of 8.33 spacing was first proposed I recall a plan to identify them by channel numbers - primarily, I think, to help to distinguish between what was assumed would be two different COM boxes on aircraft. Whatever the justification for channel numbers, the plan appeared sensible to me.
I don't know the reasons that ICAO went for actual frequencies but some of my engineer colleagues felt strongly that to call them channels was a technically incorrect and that the frequency should be specified. I can't recall why 'channel' was so wrong but one or two of them were quite adamant about it all. Maybe this view won the debate.
Maybe the use of channel numbers would have brought its own problems which don't come to mind. But many of the operational issues with using frequencies were apparent - and obvious - from the outset.