I've given several answers to PS' question, after first prefacing my replies with something along the lines of "I'm not sure if I am to be counted among the ranks of MM's adversaries".
Here's yet another reply.
If we look at the 34 original signatories, what do we find*?
First, there are the Steady State stalwarts, such as Bondi and Narlikar. It's rather odd for any of them to say this class of theories has not 'had its day in court', given the intense scrutiny of such theories in the not too distant past.
Then there's Arp. With the explicit exclusion of Gold and Bondi, I suspect that he has more papers published in relevant, peer-reviewed astronomy and astrophysics journals than all the others in the list combined. Too, he's had many grad students to work with him on his pet projects, over the years. By all accounts he is the perfect gentleman of manners ... and yet he still cannot see just how flawed so much of his research is. How to judge a fervent believer in his own ideas, when those ideas have so comprehensively been shown to be inconsistent with all the relevant data?
There are also (in addition to the above) several people who have at least one paper on astronomy (etc) published in a relevant peer-reviewed journal: Baryshev, Marziani, Paturel, and Rudnicki (though only one, that I could find).
There are the two stalwarts of Plasma Cosmology, Lerner and Peratt, who have not - as far as I know - published anything in relevant peer-reviewed journals ('relevant' in this context means 'astronomy, cosmology, and astrophysics').
Two - Ibison and Roscoe - have papers published on topics of direct relevance to cosmology (GR and MOND, respectively).
There are four plasma/space physicists (Eastmann, Heikkila, Jarboe, Orth), one 'other physics' guy (Moret-Bailly), an engineer (not EE, Ghosh), an Earth scientist (Kafatos), a chemist (Pace), a historian (Woodward), four I can't quite classify (Assis, Marmet, Neves, Soares), .... and eight who seem to have no published papers in science at all!
Now what conclusion may we draw from such a list, especially the fact that so few of the people on it have mastered contemporary astronomy, astrophysics, and cosmology sufficiently well to have had papers published (in relevant, peer-reviewed journals)? Is the bar to writing and publishing such a paper so high?
We can also ask whether this claim is credible: "Supporters of the big bang theory may retort that these theories do not explain every cosmological observation. But that is scarcely surprising, as their development has been severely hampered by a complete lack of funding." Well, it's certainly not a lack of funding that holding back the Steady State proponents! After all, many of the remaining proponents have tenured positions in well-funded universities (and how much more prestigious can you get than the Max-Planck-Institute Für Astrophysik?).
But perhaps the best illustration of how shallow this statement is comes from the reception that an alternative not explicitly listed got, MOND. I don't see any MOND proponents in the list of initial signatories (other than Roscoe). Yet MOND, as an alternative, is both newer than Steady State or Plasma Cosmology ones, and more widely accepted. Are we to believe then, per this statement, that MOND is mainstream? Or that it is not an alternative?
Or is it something as mundane as the ability of MOND proponents to write papers that meet the basic criteria for acceptance (by the relevant peer-reviewed journals), and so catch the attention of quite a few astronomers, who then go research the idea, and write yet more papers?
In short, the statement is curiously silent on what is perhaps the most important reason that the two explicitly named alternatives have failed to get any traction ... none of the proponents has been able to write a paper showing why those ideas are better - in any scientific way - than any LCDM cosmological models.
* critical caveat: these are the results of my own analyses, which are quite preliminary and may well contain errors. If you find any, please point them out.