You're just not getting this.
I'm not talking about the total
number of promotions across the two leagues, for one very simple reason - no club can qualify for more than one space.
As the league currently stands, Team A can strive for only one of the League A's 4 promotion slots. The fact there are also currently 4 promotion slots in the League B is irrelevant, as Team A can only strive for one of League A's four.
Under the new format - Guess what ? Team A could still only strive for one promotion slot from the combined league's four. Competiton for those slots may possibly be stiffer.
In summary :
Under the old format: 4 promotion slots available to each individual team
Under the new format: 4 promotion slots available to each individual team.
Iin an abstract '4+4=8' fashion, slots have been lost. But in terms of the actual impact on each individual club
they haven't been. You can't lose out on something you never had/weren't entitled to in the first place, and no team in League A has ever been entitled to more than one promotion slot from a sum total of four.