Wow I'm going to be honest and say some of the anti-football posts are mind boggling. You guys truly don't get it. Football is not the devil.
Can bulking up in the weightroom hurt being able to throw? Yes - if you do it wrong. One of the major things that allows you to throw hard is flexibility. If you don't keep up with stretching then yes you will lose flexibility as you increase mass. Sometimes certain guys are built certain ways which means they just lose MPH but overall if you're flexible you will not lose velocity.
Football is a very complicated position at all positions. Yes linemen aren't called a skilled position but if you think it's a bunch of big lumbering oafs out there then you don't understand good football. You have to hit another very large person very hard and move them while they resist. Just being big doesn't make this happen. Great agility with your feet, the ability to find leverage through hand placement along with push pull techniques and move in a small area with other very large guys occupying the same place. It's not easy and to be able to do it requires a lot of skill and just as much practice as skill guys.
Injuries - first they can happen in any sport or walking down the street. Second I will readily admit there is a greater chance of injury in football but not that much if you do things correctly. If you stick to your fundamentals, prepare your body and execute what you are supposed to do the chance of injuries go way down. Now you get injured through some fluke accident. Same with baseball or any other sport. Concussion is the main topic in football, yes it is something serious and needs to be looked at. But you can get a 85 MPH fastball to the head and get a concussion. You can take a foul tip off the mask and get a concussion. You can have a bad throw take a fielder into the path of a runner and one or both will get a concussion. Can't play scared. End of story.
Anecdotal story - I got more injuries playing baseball than I ever did playing football. Lot of people out there just like me and my point in saying this is you can't say you will get hurt in this sport over another. Injuries happen when it's time for injuries to happen. Can't stop them.
Football is actually changing from big bulk heavy bodies and to more core developed athletes. There are more and more football programs going towards core workouts because that is where explosive athleticism comes from. Havelock HS in NC has been dominating football for past few years. Their linemen are not these super huge beasts who aren't athletes. They are just slightly bigger than their skill guys. If the OL and DL guys only put on baseball uniforms they wouldn't look out of place - they would look huge - but not outrageous.
End of the day there is not a guaranteed way to do any of this. Some make it playing one sport and some don't make it. Some make it playing multiple sports and some don't. Some get injured playing multiple sports and some never get hurt. Some get injured playing one sport and some never get hurt. This is a dilemma that will never go away. We will never get to the place where we can say "this is how you get to the next level". If someone can come up with that then they will make a ton of money writing books. Since there is no one way to get there then let the kids do what they want and enjoy. If their life, it's their time to play sports, it's their journey to get to play the next level, it's their desire to do what makes them happy. Not the parents. Not the coaches. Not the private instructors. Not the best friend (although he probably has too much influence). Not the girlfriend (another heavy influencer).
I'm going to attack this from another angle as a coach. I want all my baseball players to do other sports. I do not want a single kid to be a baseball only guy. Why you ask? I'm selfish and do not want to have off season workouts. I want that time off to do other things. To go watch my guys play their other sports and support them. Because I do not think those workouts in October are going to do a whole heck of a lot to help me win in March. You put my team against a team that does the offseason workouts (talent being equal) I will guarantee my team will beat yours. Reason is because when I get them in February we will get after it. My guys will know it's time to get after it. Guys who have developed skills from other sports will pick up baseball skills very quickly. They will knock the rust off in preseason and be successful.
Think of it this way - stand on something that is about 3 feet off the ground. Then jump to the ground to where you land with both feet at the same time. Once you hit the ground if you will freeze you will be in what's called the "athletic position". Your feet will be just outside your shoulders, feet pointing straight ahead with a slight outward position and you will be on your toes with the heel off the ground. You will be bent at the knees just short of 90 degrees and hips be the same. You will have a slight lean forward and your arms / hands will be extended. This position is in every single sport there is. You just modify it for the sport. Playing other sports will help you develop athletically. I became a great catcher because of the footwork drills we did in football. Batters box to batters box you couldn't find anybody quicker than me. I could block anything but it was from football. If I didn't play football I truly don't think I would have been as good - another anecdotal story.
Let the kids play sports.