How about allowing two flavors of PATCH requests to accommodate implementations that do not wish to abuse the PUT method by employing it for partial updates?

• PATCH with application/json-patch+json: As currently specified.
• PATCH with application/vnd.api+json: Payload as in POST or PUT. Explicit intention to update the target resource with only the attributes present in the payload.

JSON API already leaves the door open for an implementation to limit PUT to its original purpose, which is to replace the target resource in its entirety. The proposed change would make it easier for services to establish this convention by simply announcing their interpretation of PUT out of two possible choices.

Right now, if a service wanted to do this, it would also need to define new rules for PATCH and would end up in violation of JSON API.