Yeah luckily these days especially MD is more about skills and mentality on the court which can hardly be trained. If you look back at the past where the points played till 15 back and forth serve ( match can technically be endless), i think there needs a lot of physical stamina. Maybe at that time, not that many tournaments like now but imagine playing back with those systems 5 matches a week up to 10 for 2 weeks. System like this many are already injured, i wonder what will happen with that system.
I don't think it plays a large role in injury occurrence. I could be wrong, but I have not seen any studies or papers indicating that the length of a sporting event has a direct influence on number of injuries. Sprinters probably get injured more than long-distance runners, although I have nothing to support that guess

The injuries are just different most of the time, shorter, more intense sports usually results in muscle or tendon injuries (especially tears), whereas longer events and the training for them usually result more in overuse injuries, e.g. inflammations and joint issues.
Kevin's style of play certainly wouldn't have been as successful in 2004, for example, but that is because of several factors, equipment being one of them as well...
The different scoring system probably just would have promoted a slightly slower pace because of somewhat longer matches, yet nothing utterly different. We still saw cramps and stuff happening with the new system as well, because it's not about the total length of the match, but rather how long any given match is compared to your usual ones, and what you've prepared for. LCW was not prepared to play several 3-game matches well over the hour mark in 2013, for example (and sceptics still point to the mysterious A/C malfunction as a contributing factor), and was right on the edge in the 2011 final as well, reportedly cramping up fiercely backstage after the match.
Sorry for the lengthy excursion. I hope you get my point - injuries will always happen in competitive sports, and no matter the format/scoring system, more tournaments equal more stress and thus more injuries. If you changed the format to something super short, players would eventually adapt and be fatigued and injury-prone at the end of a 20-minute match just as they are today at the end of an hour-long battle.