Not allowing people the convenience they want means they'll switch to a method that does. Worst case: a passwords.txt. Wouldn't that be a worse downgrade attack?
I have no opinion other than it is obviously a downgrade attack.
As a fair to middling organic language model, I cannot tell you how to keep your keys safe; I myself am blessed with a good memory and 160wpm typing speed so I use that.
Again, as a fair-to-middling organic language model, my only opinion is that, a PIN, as currently implemented in Bitwarden and described above, leaves Bitwarden users open to a form of exploitation that can be categorized as a downgrade attack.
Presumably, a better implementation would be fine.
You are free to keep using Bitwarden's PIN implementation, but it will still be open to: downgrade attack.
This is because Bitwarden's security, as described in this article, is open to: downgrade attack.
This is not an emotional term. It is no more laden with implication than observing that #F00 is 'red'. You are free to keep using #F00, just don't call it blue.
Donwgrade attacks are relatively straightforward, and ease-of-use features, such as PINs, are a traditional place to look for them.
I have no opinion on this matter other than that it implies that Bitwarden can be configured in a manner which is insecure, and the specific form of insecurity is: openness to downgrade attack.
Think it through.