The new Plus models are only available in the DS1000Z series. It makes sense to offer the MSO as an option in the cost sensitive scope range, directed towards hobbyists and students.
People who buy either the MSO/DS2000A and MSO/DS4000A series are less cost sensitive, and will buy the right model up front, based on their specific needs, with both short term and long term needs in mind.
Then there remains the question if the end user can perform the MSO upgrade himself, or whether the unit has to be sent in to the supplier or a Rigol service center.
Given that they call it an MSO field upgrade it most likely can be done yourself.