It would then become American football where offense and defense get swapped out as soon as there's a possession change. No thank you. I like soccer just the way it is now.
Hockey manages to keep the action going while substituting half of the team at once. I think soccer could manage a couple guys at a time.
Rugby used to be the same with 3 subs, but these days you can pretty much sub half the team. It really destroys finding value in superior fitness, tactical planning of the starting side etc.