So many questions are already authoritatively answered that there's a much higher bar for new users to contribute. And without new users coming in and participating, there's a feeling of stagnation. I'm not yet convinced they're at an impasse but I personally don't know how I would solve that problem.
I'm not sure it's a problem. If every PHP question you want to answer is already answered, hooray! Google solves your problems instantly. The PHP section is stagnant, but it's still helping people. The supply of answers scales down to the demand.
Meanwhile, some people start switching to Node or something else, and a new wave of Q&A starts on that. Rinse and repeat.
