In a closely divided Senate such as the one we have today, with a handful of relative moderates in each party, the presence of a 60-vote cloture threshold actually lessens the incentive for any moderate minority party (Republican) to participate on majority party legislation.
If Collins or Portman or Romney might actually like some parts of a publicly popular (one that polls well even with Republican voters) Democratic bill, and have a meaningful opportunity to contribute to their crafting, they can only benefit politically IF it actually gets enacted. They can only run on their participation in legislation that benefits their constituents if it passes and is signed into law.
Now, they will pay the disloyalty-to-party price no matter whether it passes or not, so it would be strategically prudent for them to either: 1) avoid the bill completely, or 2) join it only when it can pass and be enacted. The first scenario exacerbates strict partisanship, whereas the second is only remotely likely if there isn’t a 60-vote threshold for legislation.
The usual assumption that the filibuster causes bipartisan legislation isn’t always true. From a game theory perspective it would seem bipartisanship is best when the vote threshold is closest to the partisan threshold. 50+1 votes needed with 50-50 partisan split offers a great opportunity to see some Republican votes on really popular legislation—particularly when 7 republicans have already shown that they can stand up to their party’s groupthink already.
Have Manchin and Sinema heard this argument?