I don’t have a strong stance on any of your specific claims or your overall conclusion, but here’s one small piece of evidence that weakly pushes against the idea that coalitions are bad:
Steiner et al. (2004), using the Discourse Quality Index to analyse 4,488 speeches from German, Swiss, and U.S. parliamentary debates, find Swiss grand coalitions enhance respectful behaviour of MPs much more than the US Congressional rules and German Parliamentary procedures. Deliberative quality is highest in settings of coalitions, second chambers of parliament (for example, the US Senate or UK House of Lords), secrecy, low party discipline, low issue polarization, and the strong presence of moderate parties (Fishkin & Mansbridge, 2017, p. 10).
This quote is from the post Deliberation May Improve Decision-Making. (I haven’t read the cited studies, don’t know what the effect size is meant to be, and don’t know how important “deliberative quality” actually is.)
I don’t have a strong stance on any of your specific claims or your overall conclusion, but here’s one small piece of evidence that weakly pushes against the idea that coalitions are bad:
This quote is from the post Deliberation May Improve Decision-Making. (I haven’t read the cited studies, don’t know what the effect size is meant to be, and don’t know how important “deliberative quality” actually is.)