Everything everybody spent on cable + all ad revenue = cost of delivering cable + cost of producing all TV shows + profit
New equation:
Everything everybody spent on subscriptions + all ad revenue = cost of delivering subscriptions + cost of producing all TV shows + profit
So, "ad revenue" went from a big number to 0. "Cost of delivering" went from a big number to a small number. But the revenue went way down as well.
One solution would be to make less TV, but the industry obviously isn't a fan of that idea, so they are increasing your spend. If everybody gets on an average of 5 services and watches ads too the revenue will be similar-ish to the good old days of cable!