The Wikipedia Problem
Wikipedia attributes its volunteer problem to technology.
If it was easier to edit posts, more people would edit them.
Yet the technology hasn't changed much since its peak in 2007.
If it was a technology problem, how did they recruit so many volunteers in the first place?
Wikipedia can simplify the technology. It would probably help. More people might edit the posts. But this only adds more people at the top of the tunnel. This won't keep them there any longer. It's a temporary solution.
The problem is motivation. Why do those that spent the time to learn the technology no longer edit posts?
Too often we prescribe technological solutions to social problems. It's easier to change the technology. Move this button here, turn that to a lighter shade of green, and simplify the registration process. These things can help, but they're not a silver bullet. They mask the bigger social problems.
The real solution is usually social (or psychological). My guess is their motivation faded as Wikipedia's shiny object status faded. We know momentum is important. This decreased both each volunteer's motivation to edit posts and the number of people that wanted to volunteer.
Wikipedia never changed the volunteer commitment from desire to create something special (which faded once we took Wikipedia for granted), to an obligation to the Wikipedian volunteer community itself. The latter is more sustainable.
This problems afflicts many communities (and volunteer groups!). It's easier to recruit volunteers, contributors, and other help when your community is a new, popular, insurgent. Everyone wants to be part of it (I daresay, jump on the bandwagon).
Eventually, that shine wears off. You become part of the ecosystem, the establishment. Then the motivation dies down. We still love Wikipedia, but we're not dazzled by it.
To sustain long-term volunteer engagement you need to solve longer term motivation issues. I'd suggest the following. First, develop a comprehensive scale of increasing power for each person. This has to be open-ended, an individual should never 'cap out' of power they can have in their audience.
Second, invest money in the community of volunteers. Initiate regular meet-ups, provide freebies, organization annual gatherings, regularly seek their opinion. This needs times and investment. Your volunteers need their own special community. They need to be 'in it together'. Invest the time and money in this. It's worth it.