Someone might object. Perhaps this is the fastest route forward (i.e. directly asking for concerns).

Squeak guy here - I did a lot of prodding around the relicensing and it was pretty much the same situation. An old code base and a license that was "open source" at heart but had problematic language. It's impossible to reach everybody - Squeak has code going back to 1978 I think :-). We got the thumbs up from the main contributors (Kay and friends - in fact, I think it was Alan who started pushing for this), we got the thumbs up from Apple (that took a while but they were very cooperative and nice), we decided that Disney didn't own any of the code (that was a big relief ;-)), and then when we didn't get any objections from major contributors we went ahead and hoped for the best. Law is not mathematics, it's a human endeavour and that makes the application of the rules, however precise they're written, often more messy than we techies are comfortable with. 90% of my encounters with legal stuff is "what are the trade-offs". Ditto here - you trade off the burden of having a shitty license against the vanishingly small risk that some contributor will want to retract its code and live with the decision - if you can show you did your due diligence the impact should be low.