I'm curious that most console controllers released these days - and all handheld consoles - contain four main buttons rather than two. Obviously increases in processing power, graphics engines etc, have allowed for more complex games, but is there an argument to be made that two buttons are really enough to create some great games? In other words, would someone releasing a new handheld console on the market today be foolish not to include four?
Two are enough for some really great games. For other really great games, two aren't enough. When designing a default control system, all other things being equal, you want it as inclusive as possible.
But then - if four are better, why wouldn't six be better still? (for example, the late-era Genesis and Saturn pads had six buttons). Four's a magic number because you can have your thumb resting between them and easily reach any button without having to consider absolute position, and reasonably comfortably hit most combinations of two (for example, holding X to run/shoot and pressing A to jump). Having six face buttons isn't just 50% more complex - it means a certain way of interacting comfortably and intuitively with the controller doesn't work any more. So, for face buttons, four turns out to be a soft limit for general usage.
Contrast this with the Neo-Geo, which also had four buttons - but there, unlike modern pads, they were arranged in a line. That's because it used an arcade stick, and each finger laid naturally on each button. Going down to three buttons wouldn't change the control paradigm: going up to five would. So, again, four buttons were a natural soft limit, but here for a different reason on a different design of controller.
Why four? Because, in this context, it's not much more than two, but six is a lot more than four.
It's easier to use a button for one action than using a combination of buttons. Double Dragon for the NES made you hit A + B to jump. While A and B are also Kicking and Punching. Logically it's less intuitive if you use certain buttons for different unrelated actions. As a player you get accustomed to the layout, but it is certainly not better than having a third button to jump.
Of course this completely depends on the requirements of the game. Some games only need two buttons. But limiting game-designers to only two buttons is counter-intuitive.
If you take zelda : links awakening and link to the past for example. Link to the past had a third button to perform certain actions. While keeping sword mapped to 'B' and Item use to 'Y'. 'A' was used for various actions like pulling, talk, running etc. In links awakening you had to swap out items to 'A' and 'B'. You could even swap out your sword. Might be handy, but unnecessary if there were more buttons.
It really comes down to the gamedesign.
If a game developer chooses to use only two of the four available main press buttons, that is currently an option.
Other games may choose to utilize all four, that is also an option.
Limiting the game designer by only offering two main input buttons is simply bad design. Yes, a great game may only need a single input, but that does not mean all games should.
but is there an argument to be made that two buttons are really enough to create some great games
But there is also plenty of games that would be either impossible to make for 2 button input, or would be so cumbersome to play they would no longer be considered top X material.
In short, having few or many buttons does not mean your game will be a hit or a miss, the game itself and how well it works with the controls your target system has to offer is more important than the number of buttons the player has to use.