My kids have had other kids on their teams who do multiple sports at once, and miss games/practices. imo, it is detrimental to all involved......And I think it's kind of disrespectful to the team that the kid ends up choosing to miss the game/practice. It has been a competitive issue for our teams. I won't let my kids do multiple things unless they can commit to not missing anything based on scheduling conflicts.....which isn't likely to happen.
Here is how I see it.... I don't think it is an all or nothing proposition.
First, as for being detrimental to all involved, I have to disagree greatly with that. It certainly depends but I can tell you right now that for the athlete, I think there are tons of advantages for them being a multi-sport athlete. Being the cross training aspect that many times can help from one sport to the next in improving play. As an example, for my own son, there is no doubt in my mind that his involvement in swim contributed greatly to his success in football this year. As for being detrimental to the team, it certainly can be, as a coach I have that dynamic on my team as well as our sister teams (even more so for them) but then on the other side, if my 4 top players on our basketball team were made to choose between playing other sports or other teams and being on our team and those decided to not pay- I promise you that our team would not be the third seed in playoffs with a very good chance of winning the championship. We wouldn't have even made the playoffs- no amount of my coaching or the team playing together all the time would make up for the loss of talent. So, it is somewhat a matte of perspective in terms of it being detrimental to the team. If the players gone are truly missed then not having them at all would mean the team would be that much less of a team. Again, this is not an all or nothing thing here so it depends. For my team, some of my guys have missed some practices and games at times but at the same time they have all made it a priority to be there. Some of our sister teams had players who clearly made their other teams/sports a priority and the coaches were very frustrated. I get it.
For my son, his time at football and basketball this swim season, which were the priorities for practice time, certainly held him back on doing even better in swim that he did. Swim is his least favorite sport so that is something he is ok with even though he could really excel at it if he made it a higher priority.
As a parent, the way I approach this is that I am mindful of the commitments of our kids. We do try to limit conflicts and if there is too much conflict we will not move forward. If there is some conflict then we communicate clearly with the organizations and let them know about it and what they can expect from us. My son plays football, soccer and basketball with his school teams and then is on club swim. The school teams do not overlap at all and swim overlaps over all. I have communicated with the coaches at the swim club and they are aware, understand, accept it and support it. We looked at club soccer for him and I spoke with the coaching staff. They understood there would be conflict and were accepting of it. We finally decided that it would be too much conflict and opted to not go that route. My son also brought to me a flyer for wrestling and I shot that down quickly as again, it would be too much.
This year my daughter did both school and club volleyball and then she plays school basketball. There has been very little conflict actually. She has missed on a couple of practices from either and very little in terms of games. We left her club game after the first set of her last game to make it to her championship game for her school team. We just made the decisions based on what made the most sense. Now that school is done, there is no more conflict at all.
As a coach right now it is a 4th grade no cut "developmental" team. So, I try to give the kids equal playing time. Once we hit 6th grade there are still no cuts but it is about winning and there are tryouts for A teams and the rest go to B team/s. For the A teams, there is a higher level of expectation that the kids will make the team a priority. However, there may be some conflict. The playing time will be based on who make the team better. If there is a player missing time and it is hurting the team then their time will be reduced. If they miss a few practices but still make the team better, then they will get more playing time. The same approach I would have if a kid misses time due to injury, sickness or some other factor. I am not going to punish a kid simply because they have other things going on in their lives as well.
Ideally for athletes there are no conflicts and they can go from one season to the next but these days many sports are year round. Many young athletes have their 'top' sport and one, two or maybe even more other sports that they do as well. As with most things in life, it is about balance. You need to realize when two things may just have too much conflict with one another and then make a choice but to simply say not because there will be some conflict between sports is just too far. It limits the kids. Also, I have seen too much where kids who super focus on one sport end up burnt on it and flame out or end up with injuries because of just doing the same sport all the time.
I understand the perspective of committing to something as a life lesson. But on the other hand, adulthood is all about things conflicting and dealing with that as for priorities and balance. I think it is likely more destructive to not allow for anything else because there is any conflict.