I've watched the ebb and flow of youth keeper populations over the past 5-6 years, and noticed a few trends (mostly in the girls side, but may apply to boys as well).
Just looking at the whole youth player population, there's definitely a drop off in full-time participation right about U15. Because keepers are already smaller population, that drop off is even more noticeable. For example, if a club fields 2 teams at an age group with 32 players (16 players each including one GK each) but the next year 6 kids drop out of soccer, you could still field two teams of 13 each, but if one of those 6 players was a keeper, the second team is screwed. That creates the first shortage.
The second factor impacting the older kids is, as you mention, injury. The collisions get bigger as the players get bigger and concussion awareness nowadays means that you don't just bring your kid back to play next week after they got their "bell rung." So the older keeper population gets an additional thinning agent there, too.
One thing I noticed on the girls' side is what I called the Hope Solo effect. She, for better or worse, had a period of about 5-6 years where she was a true celebrity, arguably the most well recognizable US soccer player, men's or women's. This was right after my DD started playing keeper. Around that time, I noticed a big increase in the number of girls trying out as keepers. That was an artificial bubble, however. Not sustainable or repeatable.
The biggest factor I see in creating a GK shortage is an inability of clubs and coaches to manage the development of their young goalkeepers to the point that they want to keep playing as they get older. I've been continually shocked to watch club coaches completely mis-handle the coaching of GK's. And some of these guys are very established, very respected coaches, successful coaches. What I realized is that 99% of them are field player coaches. The extent of their coaching for a GK is "Nice save!" or "You need to make that play!" And that's pretty much it. I would reckon that the majority of GK parents (if you've watched your kid for more than 3 seasons) know more about the ins and outs of the position than the majority of club coaches. My DD has had several coaches who are so hands off about the position, that when she asked them for guidance, they just defer to the club GK coach. The club GK coach on the other hand, can teach all the technique drills in the world, but they will tell you that to integrate your keeper with the defense you need the head coach to do that, so they take a very hands-off approach to teaching tactics. This disconnect means that keepers as a whole are woe-fully under taught, and coaches take the easy way out by recruiting the most athletic kid they can find to play keeper and rely on the kid's natural ability to bail them out. Problem is, these kids generally play other sports, too. Because they are super athletic.
In short, because too many youth coaches/clubs/parents are concerned with winning games, and you can win youngers games with a big early-puberty monster in the net with no training, they invest their time in the areas where it makes a difference in the W/L column. Developing a keeper from scratch is a long, long, slow process and not worth the investment in time for a coach who just needs to finish near the top of the table to keep his team together and not get canned by his DoC.
That's my rant...
But to answer your question, yes. We have too many teams at this point. Unless youngers coaches and clubs will take a step back and re-work how they handle the position, this won't change.
