Yes, this is a product vs project argument. Products generally have a more intensive and focused effort. On the other hand the scientists which are the main users of Maxima may not be as willing to do or not be as good in development as developers themselves.
In my not so humble opinion and only in my opinion open sourcing of Maxima (or Macsyma) put it in the wrong hands. Not to blame the actual developers of Maxima. Maxima is a very good project by itself just not as good as its commercial competitors.
Maxima's user base has long been minuscule compared to Mathematica and Maple. There are many factors that went into Maxima's present state, few of which I think are directly attributable to it being open source. There are plenty of open source projects that are more than competitive with their commercial counterparts, e.g., version control, Linux in the data center, R, many MPI-based HPC libraries.
Presumably the choice was to either open-source Maxima or stop development entirely and let it fall into obscurity.
And anyway, having a(nother) open-source symbolic mathematical program is better than not having it, even if it is substandard compared to the commercial offerings.
Take a look at Maxima then. It is a good system too but not even close in quality, features, etc.I blame its open source nature for this.