Chris Bowers (who seems to be blogging a lot now, which is excellent news) argues that the main obstacle to Senate reform has been senior Senators -- and that therefore cohort replacement will eventually produce much more significant reforms that what the Senate managed to enact last week. He has a nice chart which highlights the role of senior liberal Democrats in opposing Merkley/Udall.
The problem here is that we don't know whether folks such as Barbara Boxer and Carl Levin are reluctant to support reform (or perhaps reluctant to support majority-imposed reform) because they share values and norms of a disappearing Senate -- or because Senators tend to evolve in that direction as a result of long service in that chamber. If the former, the Senate Democratic caucus might be purely pro-reform in ten years; if the latter, a different group of senior Senators will be reluctant to move quickly on reform -- if though they supported it this time around.
And there's a third possibility. It's possible that the senior Democrats who resisted reform were in speaking for many junior Democrats who found it difficult to take that position publicly.
Of course, the other part of this -- perhaps the most important part -- is that the partisan context overrides most of this. I strongly suspect that had Democrats had only mild setbacks in the 2010 elections and then emerged from the 2012 elections with unified control that they likely would have implemented more substantial reform last week (if not earlier). On the other hand, while I don't really expect Jeff Merkley or Tom Udall to flip, I would guess that most Democrats will turn anti-reform if Republicans take control of the Senate in 2014 or 2016 -- and most Republicans will turn pro-reform.
I do expect the next Republican majority to be somewhat less hesitant on reform than the current Democratic majority. On the other hand, Republicans did not, in fact, use majority-imposed reform when they had the Senate during the George W. Bush presidency (although they did threaten it in order to get judicial nominations through by simple majority). On the other other hand, however, Democrats didn't enforce a full 60 vote Senate back then; they might, following Obama-era Republicans, do that next time.
Meanwhile, if Democrats survive 2014 with little damage and then win a landslide in 2016 -- especially one that leaves them with united government but a few short of 60 Senators -- then I think majority-imposed reform (or very significant reform driven by the threat of majority-imposed reform) becomes very, very, likely.
Which is only to repeat that partisan context matters more than cohort replacement.