There's also uneven concentration of the open-source effort.<p>In the past 15 years, companies, or at least the companies I worked for, loved open sourcing their systems for better reputation, for larger community support, and for not getting left behind by competing solutions.<p>I sense that the tide has changed. Now we have a lot more mature solutions than a few years ago. Few companies are growing so fast on a new domain that calls for brand new systems, either. So, the cost of open-sourcing a system may outweigh the benefits. Nowadays, it is more often than not that a system startup open-sources their systems to gain user traction, and soon makes their systems only source-available. And rightly so. They have a business to run anyway, and they certain don't have any obligation to open source their bread and butter.<p>There is another factor: the fun is always in new challenges, yet a lot of software development has more chores than challenges. I remember many years ago a pretty popular blog explained why Linux did not have a nice UI compared to Windows or MacOS. The main reason, the author argued, was that tweaking UI was not necessarily fun, and we achieved great UI from thousands and thousands of incremental improvements, more often than not painstakingly. An engineer who worked on open source in her spare time most likely did not have incentives to make such improvements. I mean, what's the fun in that? In contrast, people work for free to create amazing libraries in the field of deep learning. I'd venture to guess that's because there's tons of fun in working the "sexy" part of the engineering: deep understanding of optimizations and healthy dose of mathematics, chances to apply or even invent all kinds of engineering tricks, very cool results, potentially huge impact, cheering from a large community, and etc. Such projects are deep, are personal, and are super fun to work on. On the other hand, I'm not sure how many new fun projects (at least in the area of distributed systems) are out there that have outsized impact to the community, compared to 10 years ago.