I don't buy the first argument - firstly, I think three subs would cover most bottom half clubs' key players, and secondly in a scenario where the game is all but over, the tempo of the game usually slows down so much that it's comparable to a training session.
Of course more subs provides more opportunities for time wasting, but that's a tiny advantage compared to what the big/rich teams will get out of this. More subs will contribute to a further cementation of the top teams' dominance, because they'll have more quality options available to beat those teams who doesn't benefit from financial doping or the effects of historical dominance.
IMO small/poor teams might get equal opportunities from five subs while playing each other, and the same with big/rich teams - but when big/rich teams face small/poor teams, five subs is a massive advantage for those with superior resources. I.e. clubs that already are in a priveleged position.