I like MongoDB a lot, and the improvements suggested would really strengthen the product and could make me more comfortable to use it in more serious applications.
Like the article suggested, it would be one thing if they did it for transaction support. In reality, from looking at the code, it seems like the global write lock came from not wanting to solve the hard problems other people are solving.
Adoption is hard to replace. Modifying API's is really hard. Rewriting a core engine? Reasonable and in this case probably necessary given the issues.
There are lots of people here who could replace that core engine and someone should if the MongoDB guys can't.