That's all I can think of too, but it seems like something else has to be involved. It looks like playing length has been in free fall starting around 2008-2010. Something different had to happen to explain this.
Players have been getting better since sports were invented. I guess you could say its reached a tipping point and injuries/awareness are causing players to opt out but to me (without any data) its seems like teams are the ones responsible for players not playing in monumentally more significant numbers.
Although it seems very reasonable, has also been put forward, that there are more quality players available now. I don't think that is something new either. As more people play football and it gains in popularity the bell curve gets bigger and more people are in the middle, this should have a relatively continuous effect.
Perhaps its the new CBA that is to blame. Although the decline started around 08, that could definitely be explained by natural fluxtuations. Things didn't really start getting significant until 2010, perhaps precipitated by the lockout and new CBA. Were there any major changes that would stop teams from signing there own? I don't really know the CBA but it seems like this has to be something new, something that was changed.