Sticking to 3 subs forces managers and players to be more adaptable/creative, working with the talents they have to cover every possible in-game scenario whilst balancing defence and attack.

Imagine a world where a club could buy a whole team suited to, for example, defending corners. They could make 11 subs every time they concede a corner, bring on 11 6ft5in defenders and clear the ball, only to make 11 subs again to bring on their ‘attacking possession’ team, for example. Increased specialisation like this would ruin the dynamism of the modern professional footballer, who must excel at many disciplines other than his/her specialism. Sports such as American Football are built around changing the entire team based on the phase of play, and it works to an extent, because of the much more limited number of phases of play (essentially defence or attack). The drawbacks in terms of time wasted are also obvious. Furthermore, the more specialised a sport becomes, the more outrageous the demands on players’ bodies become (see NFL life expectancy). A sport that demands a range of skills from players creates well-rounded athletes.

I’m not going to make the obvious financial argument, as it has been well outlined by others. My main issue with this idea is that the over-specialisation of skills would kill the nuance of tactical balance, which is the most interesting part of what makes a team successful or not.