It's probably appropriate that in the debate about Senate procedural rules, it turns out that the procedure followed to change those rules is far more important the substance of the actual changes brought to the rules. No, really. If Democrats manage to hold together and force a rule change on a simple majority of Senators a century plus long fever-dream delusion of the US Senate will have finally been broken. A Rubicon will have been crossed. The rules will have been changed by 51, and this can now happen again. It won't matter anymore if the specific scheme they adopt works. If it doesn't, they (or a new majority) can change it again in the next Senate (or during one, but let's break down one imaginary Senate Les Nessman office wall at a time).
With that in mind, this now makes perfect sense:
The problem is not that filibuster reform won’t happen — if it came down to it, Dems would likely be able to mobilize 51 votes to pass reforms via a simple majority. Rather, the problem is that Democratic reluctance to go for this “constitutional option” is causing them to lean more towards negotiating a deal with Republicans — enabling it to pass without the constitutional option — that risks diluting reform. In other words, even if Dems can pass reform via simple majority, enough Dems may end up preferring instead to reach a deal with Republicans on a less comprehensive reform package.Think about that. They have the votes for reform on their own, to do what they want to see done, but they would prefer as a matter of deep principle, to water down their reforms in order to win significant Republican support. This isn't about valuing bi-partisanship as some kind of end in itself, it is about maintaining the shield protecting the Senate from real majority rule.
This is the real fight of filibuster reform. When you wonder why Reid and Merkley had trouble mustering 51 votes to reform the thing after 6 years of absurd Republican abuse (remember, the ramp up in filibusters started as soon as Democrats took the Senate in 2006), this is the real reason. They don't love the status quo, but they know that any rule change done by 51 will spell the end of supermajority politics in the Senate generally. One way or another it will end the filibuster.
None of this is unprecedented. The "Gang of 14" deal was reached by Senate centrists in order to avoid the GOP using the "Nuclear option" - Democrats decided that it was better to almost entirely cave on Bush's extreme Judicial nominees just to avoid breaking the seal and having any substantive change to Senate procedure happen on a straight majority.
From what I gather, the rule change that took place in 1975 (where cloture changed from two-thirds to three-fifths) was the same. Once everyone understood the majority was prepared to act, suddenly a much weaker deal was found so they could all maintain the elaborate self-serving fiction that 67 Senators are actually needed to change the rules. This is why the 1975 rule change failed, and in fact ultimately made filibusters more common and harder to defeat.
It really looks like this round of reform will fail. It will either not happen, or some useless tweaks will be enacted by 67+ Senators. This will only change when liberal activists understand why Senators behave this way, and get serious about demanding commitment to real reform in Senate primary candidates. Rank-and-file Senators love the filibuster. It's bad for the country, but empowers them. They're not going to give up individual effective veto power over the nation's business without a fight.