What Senators Manchin and Sinema seem to ignore is the fact that voting to rescind the 60 vote cloture rule doesn't automatically sign them up for every legislation spat out from the House with support from remaining 48 Democratic Senators in the current Congress (The Confused 117th!).
These two senators - for those who may not know - are the obstacle between Democrats and the end of Filibuster as we know it. The ostensible rationale for not siding with their party on this issue is that this will essentially abolish the last rule standing that encourages bipartisanship.
This is notwithstanding the extended term lengths of 6 years and the election cycle for only a third of senators every two years, which were the instruments implemented by the Founding Fathers to make the upper house a more deliberative body than the impassioned House. Filibuster, on the other hand, isn't from the minds of the Founding Fathers. The two Senators seem to ignore these realities as well.
If we imagine a Senate with no 60 vote threshold for passage, then still only a unified majority party can move the bill to a passage. That is, the Democratic party would need the backing of the entire caucus to pass a legislation. And such a legislation that receives the blessings of centrists like Democratic Senators Manchin and Sinema are indeed worthy of the passage. After all, an equally divided Senate is a mandate for progressives to find common ground within their party and move the United States a bit to the left. A bit because Manchin and Sinema will not let it swing wild.
Now imagine a majority of 54 Democrats with no cloture rule. This would be a mandate to steer the ship aggressively towards left. Even if Manchin/Sinema pair dissents, Democrats will most likely still have the votes to move forward. You get the point!
This is exactly what the Republicans did when they were in the majority. For their tax legislation, they (including President Trump) came together and passed it with zero, zilch, Democratic support. Did McConnell stop for a second to question the partisan nature of the legislation? Of course, not. (Is he one of the longest serving, most consequential, Senator and Majority leader in the history of American Government? Undoubtedly, yes.) Granted they didn't need to because of the reconciliation rule that allows certain fiscal bills to pass with only a simple majority (51 votes). But neither do the Democrats need to keep the Filibuster.
You see Republicans are content with keeping filibuster even when they are in the majority. Their agenda is more often than not fiscally oriented and, guess what, it's covered by the reconciliation. So, each fiscal year during which they hold the majority, they can come up with a mammoth omnibus bill, stuff all their pipedreams and pass it without Democratic support. They never, ever, care for it - including for the support of Manchin or Sinema, for that matter.
Democrats, however, are concerned with social progression which deals with strengthening of social welfare for the poor and giving rights to underprivileged. Whether you agree to such an agenda or not, they are costlier! Hence, reconciliation process doesn't cover them! So what you end up seeing is desperate acts from Democrats to jamming social reforms for undocumented immigrants, for example, in the fiscal budget with hopes that the parliamentarian will not strike it down (which she will!).
So, what does this all come down to? If Democrats don't end the Filibuster, other than passing a slim legislation to legalize DACA recipients (even this is a big maybe!) they really won't be able to pass any flagship progressive agenda. They would have failed. And McConnell is going to remind everyone of that come 2022 election cycle.
