First off, the MOOCs associated with this degree will still be free. The
degree is not. The change here is that a) an entire MS worth of course material (and the systems to support it) from a respected institution are being created, b) the actual (O)MS degree just became a great deal less expensive.
It remains to be seen whether this lowers the bar or merely opens up more opportunities. I don't think education is a zero sum game. And while there may be a glut of PhDs, there is a shortage of employees with the advanced knowledge and skills in demand now. That's why e.g. AT&T is pushing this program.
While the union issue and age discrimination are interesting and important topics, CS graduates don't have a lot to complain about economically. The field has maintained high employment and excellent compensation during recent times when many other professions suffered greatly. Now, at 40 I don't expect to get funded by Y Combinator because I've almost certainly suffered crippling age-related innovation dementia, but there are plenty of employers that don't think that way.
There have been several articles posted here about the glut of PhDs versus available faculty positions. And actual instruction is a pretty small part of what full professors are expected to do. If you aren't pulling in research grants, your career is on a weak trajectory. So the problem was already there, and I don't think this seriously affects it one way or the other.
One might hope that making education available and practical to many more people at much lower costs might actually help advance the field, both within and without the ivory tower.