It looks like even if a forum approaches this happy state of affairs, the way old Overcoming Bias and early LessWrong arguably did for some time, this can happen only as a brief and transient phenomenon.

Are you concluding too hastily about the cause of deterioration? In the early days, OB had two major voices with conflicting ideologies. I think that's what lent it greater intellectual excitement. I do not think it a matter of ideological alignments being absent in the golden age. Rather, it allowed space for discussion of fundamental differences--as opposed to the analysis down to quarks of the highly obvious that's taken front seat today.

Consider this old posting on OB. The level of objectivity wasn't higher, but the level of engagement with fundamental issues was.