Realization
Imho it's far simpler than most people think.
The reason why open source is not growing anymore and is actually shrinking, because by now even the most zealous fans have realized, that it was all built on a false premise and offered false promises. OS doesn't really make the world a better place, it just allows others to take advantage of those who have put time and effort into it, without receiving any payment in return. Or if they did, then the whole point of that open source development was to displace competitors and inrcrease demand for complementer products.
Also the source itself is being available is less and less relevant with online services and big data taking over from locally run software. The role of the software itself has become marginal in most cases, and the service and the data are either locked down to begin with, or is not even feasible for everyone wanting to store and process them themselves. Like what use do you have for your media player or codec being open source, if the content you could play with it is only available through online services and takes up petabytes and exabytes of storage? What use is it if the social media app source code is available, if there's only place for one or for a few central social media hubs? What use do you have for open source AI models, when training them requires 1000s of high-end accelerator cards costing >$10.000 each, downloading and storing petabytes of training data, and the resulting model weights are not available openly as is the source? Etc.
Some of use have realized this decades ago - after we have also put several years of hard work into OSS projects -, others are realizing it just now. Open source mostly benefits behemoths like Google, Facebook, Apple, etc. and the most a small everyday guy can get out of it is not having to pay $50 or 100 for a Windows licence. The average developer, who put 1000s of work hours into OS project is even worse off, and is in an actually net negative of several hundred thousands of dollars (when comparing how much he gained through OS, and how much he put into it).
Those who still do open source mostly do it, because
1. they're paid for it (in which case it's not really open source, as originally conceived, and again merely a business tool to disrupt the market, crush competition or generate demand for non-open-source services, parts, etc)
2. they still didn't realize all the above, or are just unable to admit it to themselves. they're considering only their sunk (personal time and effort) costs, and don't want to give up on the (false) idea, because that would also mean admitting to they having wasted large chunks of their lives on said false idea.
The latter people will be slowly too old to contribute and even possibly die, and the former will still be around, but they're not really making open source stuff, and with said rise of the cloud and data the source being available will be getting even less and less irrelevant, than it is today.
I know this post will not be popular (especially amongst "believers"), but it's still he truth, unfortunately.