At what point...

[ALR-bishop]

Not aware of any requirement. From what authority would such a requirement originate ? My assumption has been that the "need" to sell the card with something else was originally market driven, and then came to be expected by that market, and that once Topps established that the "market" expectation was gum, attempts by others to break that expectation did not fare well.

After 1981 when Marvin Miller had helped break the Topps exclusive gum/confections contract, everyone started using gum. Then manufacturers had to find other things to put in the packs to distinguish themselves for those buyers who wanted more than just the cards.

Even during the heart of the Topps monopoly period in the 1960s and early 70s, it often included insert items with cards to help stimulate sales