That is the catch with reducing the number of adventures: it's also reducing the number of potential adventures people want to play.
Regardless, not everyone event wants to play pre-published adventures. So mixing it up with more products and relying on the back catalogue for new players is sound.
The catch is you only every need one or two published campaign settings. Unlike splatbooks where the entire table might make use of two or three different books, you're not going to run a game that uses more than one setting. A single setting has material for multiple campaigns, years and years of play. And since they're not going to go out of print, they'll just end up competing against each other on the shelves.
The first setting will sell well. The second will sell significantly less as fewer people need it. The third will likely be ignored unless it's something radically different.
One campaign setting is a good idea. More than one is redundant and unlikely to be used.
The further catch is that campaign settings don't really have a strong audience. The new players (and man is there a lot) have zero affection or preference for any of the settings published twenty to thirty years ago. The established fans of the settings already have the books and aren't guaranteed to buy it again.
Plus... there's already a best selling 5e campaign setting in stores: the
Tal'Dorei Campaign Setting. WotC will have a hard time competing with that.