The problem is "client credential" flow is not always available... And even if it were it might not as easy as you claim. In particular if you want to have an access token working on several instances. Most implementation allows only one access token at once so you need to have a shared storage and synchronization just for that...
If you are developing an API please have mercy: provide something other than OAuth. For me, avoid OAuth unless there is 3 distinct parties.
If you are developing an API please have mercy: provide something other than OAuth. For me, avoid OAuth unless there is 3 distinct parties.