In 3.X, there was a "standard" of 4 players and 1 GM in the game. CR was based on 25% of your resources being used in an balanced CR/Party fight. It was still up to the GM if that 25% meant 1 PC death or 1/4 of everyone's hit points/spells across the board.
Yet in most of the games I have ever played, 5 was more of average, with a 6th being common in 2e days. Hell, most of the time if there are "only" 4 players, we start making phone calls.
So if you take it that a designer is making a game for 4 players, that means the level of complexity and length of combats are angled to that number. Perhaps most GMs are over taxed with 3.X feats and 4.0 powers because they're running larger games.
On the other hand, if you designed 5.0+ to run with 5 or 6 players, that means the designers need to lower the complexity of each PC build and find ways to make rounds go faster with more people.
So you can still run with only 4 players, that just means the fights go faster and the PC are more streamlined. How is that not a win/win?