That said, I generally don’t have it on because of compatibility reasons with older code.

I was just wondering which older code is not compatible. Box modules? Or your own non-module stuff? If people are not enabling null support we will never discover null support bugs in some of the more common modules. Unless we built separate test with null support enabled and try to target some stories where null values can occur.

Not looking forward to sprinking if (isNull(foo) || foo == "") everywhere...

I mean you could put it in your app's rules that there must be null support in any modules being used. Put your foot down haha!