I can’t speak for Petr, but what I meant with overlapping terms improving diversity in voting (rather than candidates) is that right now, I’m reluctant to vote for just the people I think would do great on the SC, without also voting for the ones I know will be great. Because we use approval voting, the known-good people are more likely to get more votes. (This has been discussed extensively before, but approval voting isn’t that good for selecting diverse voices for (at least) that reason.)
If we use two overlapping terms, I can look at the SC members not up for re-election, see that there are two or three known-good members there, and decide to vote on just the ‘risky’, ‘different’, ‘outside’ candidates.
I do agree that a longer commitment impacts diversity of candidates, but I think it’s outweighed by the diversity in voting, and considering the stability we (at least I) would like from the SC, I don’t think a 2-year (soft) commitment is entirely unreasonable. (We currently ask for a 3-year commitment from Board members, and a ~7 year commitment from release managers, after all.) As Petr said, there’s also no problem in stepping down after a year, or even saying so up front.
Another aspect of overlapping terms is that it can make it easier to decide to not run for ‘obvious’ candidates. I am currently one of the most senior members of the PSF Board (which 3 overlapping 3-year terms for its 11 members), and my term is up this year. I decided not to rerun for the Board and make room for someone else, and that decision was a lot easier because I know who 2/3rds of the Board will be regardless of which Board members do rerun or get elected. I don’t know what I would have decided otherwise.
I also agree that we should document everything and figure out fixed procedures for ~everything the SC does (which we’ve been doing), but even with all that, in my experience it’s been very valuable to have SC members to provide the context missing from notes, or the unwritten reasons for particular choices, etc.
(There are other ways we can try to address the diversity of votes, like using a form of Single Transferable Vote, like the OSI has started using for its Board elections. That is, however, a much bigger topic, and I don’t think overlapping terms precludes that discussion.)