The existence of Theora, VP8, VP9, and now AV1 seems to contradict that theory.
You could argue that they infringe on some unknown patents, but that is also arguably true of patent cabals like MPEG (you just hope that the cabal is big enough that there aren't any patentholders lurking outside). The only difference is that with a patent cabal you have the fun of having to obey the restrictions of everyone who showed up with a possibly-related-in-some-way patent and joined the cabal.
Not to mention that it isn't necessary for a patent pool to be a cabal. AOMedia has a similar structure to a patent cabal except it doesn't act like a cabal (its patent pool is royalty-free in the style of the W3C). So even if the argument is that a patent pool is a good idea (and video codecs cannot be developed without them), there isn't a justification behind turning the patent pool into a cabal.
> At least the patent licenses usually used with MPEG mean that private use of open source implementations is free.
You say that, but there's a reason why some distributions (openSUSE for one) still can't ship x264 (even though the code itself is free software). Not to mention the need for Cisco's OpenH264 in Firefox (which you cannot recompile or modify otherwise you lose the patent rights to use it). The existence of the MPEG patent cabal isn't a good thing, and any minor concessions you get from them do not justify their actions.