Hello, I think you make a good point, about the necessity to carefully weigh the up- and downsides of each system.
I do not have a strong view on which alternative voting system is best, since I haven’t looked into it deeply enough. Still I want to address this proposition:
Much more is gained by displacing plurality than is lost by replacing it with a suboptimal alternative (for all reasonable alternatives to plurality).
I mostly agree with this position, especially in scenarios where no other option is realistically on the table. However, I do want to point out, that adopting a sub-optimal system can have a considerable cost and that it is not entirely obvious that this cost is irrelevant relative to the gains obtained from switching away from the status quo; in particular, if one believes that the difference in outcomes between two alternative voting systems is big.
For instance, one might assume alternative voting system B to lead to much better results than system A. If this were the case, then switching to A (the weaker system), though (probably) better than the status quo in itself, could still lead to outcomes that are worse than if the switch had not happened. This is for 2 main reasons:
First, as Tobias points out, countries do not change their voting system frequently. Hence this sub-optimal system A might potentially stick around for a century to come, before maybe being changed to the better alternative B. It might be preferable to postpone the switch by a few years, hopefully increasing the odds of switching to B instead of A.
Secondly, this new system A will inevitably be questioned by the electorate and the media. If system A then yields controversial results that are not obviously better than the results one would have got with the status quo system, the whole switch might be viewed as a mistake by the general population. This might even lead to less trust in the political system, though probably only in the short run. Still, a negative experience of this kind, may not only have short-term bad consequences for the country itself in the form of further erosion of trust, but could also discourage other countries from switching away from their respective status quo system for years to come.
Of course, I’m not arguing that switching should be postponed until absolute certainty of one system being better than all others is reached. (That point will probably never come.)
And, of course, I also acknowledge, that the opposite of the described scenario might happen, i.e. that one country switching might encourage others to do so, rather than discourage.
All I’m saying is that there is a case against switching and that therefore, not any system that seems preferable to the status quo ought to automatically be endorsed.
Thanks to your comment, I can now endorse what you said as a more accurate and nuanced version of the position my previous comment tried to articulate. Agreed 100%.