Like which countries?
I am not sure how non transferability of these rights work in combination with authors right to permit those things. What exactly happens if author permits modifications of work? Does it only apply to specific modification and each modification needs to be permitted separately? If not it would seem contradicting with this right not being transferable.
I'm only familiar with Germany because I live here, but from talking to a friend about this recently, the situation is apparently similar in other countries as well. They were looking into using OSS and ran into that issue because they'd also need to look into the (at the time of contribution) home-countries of contributors to make sure that the license would "stick".
Say you work as an anonymous contributor, and all the evidences which could proof the fact got accidentally destroyed, isn't that effectively relinquished your author rights? (Moved the right away from you, the right could still be there but licensed under an non-existing entity)
No, that's just making it difficult to prove you had the rights, not relinquishing them.
However, it gets complicated quickly. For example if they decide to sell the company or take on a new investor, the author could take back the grant under some circumstances which brings its own set of challenges. Can you now simply fix it yourself by writing the same code? Either you'd violate copyright or you didn't need the whole licensing thing in the first place.
Another issue is the fee: you can agree on a fee but if it's not "appropriate", that agreement is void (to protect authors from being tricked with regards to the value of their contribution).
There's a lot of uncertainty involved, which is why companies like using commercial licenses: you can usually trust that the company selling that license has figured out all the details.