Well, the operator wants a machine that will make money and the designer needs to support that. The designer wants to create a game that people will want to play and keep playing. Which means a theme that attracts them to try the game, and then game play they enjoy, find challenging and draws them back by either providing exciting play or establishes challenges they want to strive for. If the designer succeeds in those things, then the operator will make money, and the manufacturer will as well as more of those games will be sold and the designer will get to design more games. Game objectives, like wizard modes, are supposed to keep you coming back to play more. They need to be designed to be achievable (an for the player to feel they are), but at the same time require the player to keep playing.
However, ball and game time is certainly a factor for an operator and for how the game is designed. If EBD was a 3 ball game when it came out, it would not do well, players would feel they the games were too short, that there were 'cheated' and they would go put there money elsewhere. Nobody wants a game to be over in a minute or two, and few if any want a game that takes an hour. I think most would agree that the DMD era games have sufficient ball/game times at three balls, with a few exceptions, that also meet the criteria to keep a player entertained, challenged and coming back, and it will let you, with skill and time, achieve the more advanced objectives. The three ball games of the DMD era, take longer to play then 5 ball games of the 60s, 70s, and 80s. Seems like three balls on those game is what is right.
Like most things in life, it is all about balance. A game that balances all of these things will be successful for both the operator, the location player and the collector.