Require a specific number of Senators -- I suggest five for the first 24 hours, 10 for the second 24 hours, and 20 thereafter -- to be on the floor to sustain the filibuster. This would be required even during quorum calls. At any point, a member could call for a count of the senators on the floor who stand in opposition to the regular order, and if the count falls below the required level, the regular order prevails and a majority vote is held.This is in the tradition of reform proposals from Jon Krasno and Gregory Robinson, which also try to make it harder to filibuster for a group that has enough votes for cloture -- they would use frequent cloture votes and require an affirmative 41 votes to keep a filibuster going (as opposed to 60 to invoke cloture).
I more or less agree with Merkley (and Krasno and Robinson) that it makes sense to place more of the burden of carrying out a filibuster on the opponents of the bill. And as far as I can see, Merkley's proposal would be a real disincentive to filibuster. However, the devil is, as always, in the details. Essentially, Merkley (and Krasno/Robinson, on this point; read their whole proposal and my response) would raise the physical cost of filibustering on minority party Senators. That might make filibusters somewhat more rare.
However, the main reason that live filibusters died isn't because of the parliamentary situation; it's because floor time is valuable to the majority. Given Merkley, the minority party could still easily stage a live filibuster if necessary; all it would take is for 20 of the 41 (or more) filibustering Senators being willing to sit on the Senate floor. And in an era of strong partisanship including a partisan press, and an era in which many Senators are as worried about primary challenges as they are about general election votes, it's not at all hard to believe that 20 could be found.
And if that's the case, then Merkley doesn't actually change anything. Right now, Harry Reid could force a "live" filibuster on any issue. He doesn't, not because he doesn't think that 20 Republicans would participate, but because the logic of the live filibuster favors the minority.
I don't see why that would change under Merkley's rule. Greg Sargent says that Merkley would make filibustering "much more politically difficult than it is right now." I agree it would make it somewhat more physically different, but politically? The constituencies that matter to Republicans who oppose Democratic legislation are, well, opposed to that legislation. Tom Coburn and Jim DeMint are not afraid of the constituents they care about finding out that they oppose DADT repeal, or New Start, or even food safety -- they would love the chance to have their opposition covered by the major news networks, even the ones that aren't Fox News.
So I'd say that the Merkley proposal would succeed in slightly tilting away from filibusters in marginal cases, but mostly have little or no effect. Mostly, I'd say that it's a sign of the general breakdown in Senate rules and norms. Proposals are proliferating. Change is coming.