Since mitochondria are certainly a good thing, it follows that harming them is bad, yes. So the question is, how much acidity caused by glycolysis will harm them? The body is certainly made to handle a certain amount with no problems. How much is too much probably varies a LOT from one person to the next, depending on variables such as aerobic condition, muscle mass and fiber types, overall health, hormones, other stressors in life, and genetics. Then there are the variables in the exercise itself: duration, intensity, rest periods, etc. So in general, if you're going to repeat something day after day (like S&S swings), it's best to do so in a manner that stays on the safer end of this spectrum. Or if you're choosing what exercises to do, it's best to spend time doing LSD aerobics in a MAF-type HR (low intensity) and high intensity power work like swings and snatches in shorts sets with plenty of rest. Pure strength work follows its own rules, but again, short sets, plenty of rest are good (depending on your objectives). In contrast: a 5-min snatch test every day, or a CF metcon every day, or a maxed-out spin class every day, or a HIIT class every day... you get the idea... excessively training the anerobic glycolytic energy pathways, potentially leading to adverse effects on mitochondria, greater injury potential, excessive stress, and overtraining. Best to do this type of training occasionally, or when peaking for an event.