Not sure if you are suggesting this default be for just observations or both them and photos.
This was covered in a very long thread here
https://forum.inaturalist.org/t/a-case-for-changing-the-default-license-to-not-include-a-nc-clause/18690/84
For observations I really have no opinion since you can’t copyright a fact anyways, the issue here is people cutting and pasting already copyrighted text and then ‘relicensing’ it.
For photos, I 100 percent disagree. Too many users would be somewhere between upset, angry or surprised to find out they have licensed away rights to their photos via a open license. If a user wishes to do so, that’s great, but they should be the ones making that decision and actively implementing it. Not having it made for them in something few users review or read.