I am okay with kids focusing on one sport in high school. Most of the folks who study this issue consider 15 or thereabouts (for most sports) to be a fine age to focus. You can specialize in one sport and still play another.
What bothers me more is the coaches who place a lot of pressure on athletes to specialize in one sport. I think in the days when coaches were often PE teachers (either first or in addition) they understood this. Now that HS coaches in a few sports are earning a lot of money the pressure to win is greater so they push that down to the kids on the team.
There is also the matter of the size of school and town you live in. There is a lot of data out there showing that kids from smaller communities make the pros at a higher rate than those in big cities. I make the argument that those athletes in smaller cities go to smaller schools so they play more than 1 sport. If you are the best athlete in a school of 400, your friends are going to cajole (maybe it can even be pressure) you into playing other sports.
https://www.tandfonline.com/doi/abs/10.1080/02640410500432490I know in team sports college coaches like the kids who played more than 1 sport. Football is the easiest mainly because due to the nature of football year-round play does not happen. Even the 7 on 7 leagues are not that big of a thing and plenty of kids on those teams also play baseball. One reason I heard from a coach is that they know those kids are more coachable because they have had to work with different coaches in different sports so the transition to learning from a new coach in college is easier. It can also mean having to learn to interact with more people on a team (assuming not all the players on sport A are also playing Sport B). There are also more opportunities to learn read and react to movement (agility if you will).
I think it was Johnny Majors at Tennessee liked linemen who wrestled and/or were throwers in high school. Wrestling taught leverage. The throws in track and field taught footwork (incredibly important for an offensive lineman in particular).
From another perspective, it gives the body a break from the movements of one sport and reduces the risk of overuse injuries. Runners are highly susceptible to overuse injuries since running is a repetitive motion sport.
One issue comes into play for a lot of runners: what other sport do I play? This is where it needs to be emphasized that the off season sport does not have to be as competitive as your primary sport. Join a rec league or church league basketball team. Something low key where you might practice a few times and then just play games. You can still run some during the winter. Another option is swimming.
Another option is to do an activity. Ride bikes, paddling a canoe/kayak, ski. Being active does not mean you have to be in an organized sport.
Sadly a lot of distance runners land in running because it is the sport of "last resort" (other sports have this reputation as well such as fencing). They might be small. They might not be quick enough for most team sports.