Sometimes people can't contribute back to the projects (lack of time, lack of wilingness to create a fork + put a 'marketing page' up and convince others...).

Constructive criticism (like in this article) is a valid way of trying to convince others that a better way is possible. He is cleary stating the use cases he would like the tools to fix.

Shooting down the critic by saying 'so fix it yourself/so fork off/so go code another lib out of the 30 000 that exists' is, on the other end, not constructive. Do you have better arguments?