First, I want to make it clear that my above criticism for the learning material stems from my personal experiences trying to learn MonoGame. A couple of years ago, I had grown frustrated with the black-box nature of GameMaker and wanted to move to something open-source. I’d heard great things about MonoGame, but I found the website unhelpful and confusing. Most of the provided documentation was just linking to tutorials that used outdated versions of MG and didn’t really teach anything beyond the basics. I tried looking into the XNA examples @IceIYIaN mentioned, but as a complete XNA noob it wasn’t clear to me how to port them to MG. So I just gave up for a while until I found a good book on XNA that walked me through the basics step-by-step and taught me how to make a moderately complex game with XNA itself (not using MonoGame). That was really the key turning point for me. From there I could contextualize what I had learned into MG.
This is just my personal experience and opinion. If you were able to learn everything you needed from MSDN and community forum posts, great! That just wasn’t the case for me, and I imagine that there are other potential MG users who feel the same way. New users aren’t necessarily going to have the instinct to look up XNA resources, or even if they do, they don’t necessarily know how to use them. Hence my suggestion to improve the documentation on the website itself. Even just updating the list of tutorials on the official docs page (www.monogame.net/documentation/?page=Tutorials) to more recent tutorials would do a lot of good, I think.
The two MonoGame-Samples repos you mentioned haven’t been updated in years, by the way. While I think we both know that the material is still probably applicable to modern MonoGame, this may not be obvious to newcomers, especially if they’re used to frameworks/engines that have many breaking API changes between versions (like I was).
Finally, in regards to the Udemy course situation, I am aware that there are literally hundreds (555 at last count) of Unity courses on the website. Most of them have less than a hundred students, and several have single digits. But there are also many courses with thousands of students. In contrast, MonoGame has 3 courses on Udemy, and they all have < 200 students. What I take from that is those low student numbers is that: If lots of newcomers wanted to learn about MonoGame through a paid online course, they would buy one of the few existing courses. But they haven’t, at least not in large numbers. So I’m not sure if a paid online course in the Udemy format is the best way to reach out to new folks and further educate existing users. There doesn’t seem to be enough interest in the existing courses to justify making new ones. (Now, YouTube is a whole different story. Many MG tutorials have tens of thousands of views. Maybe it’s just that people who want to use free open-source software don’t like spending money? 
)
I agree, though, that there are definitely situations where an advanced, paid course makes sense. (Deep dives into niche features, etc.) Or maybe more comprehensive, beginning-to-end courses for learning beyond just the basics. Again, though, the biggest challenge is making sure that there is a sufficiently large interested audience who are willing to pay to learn the material.
