??? The sim has been open to 3rd party developers from the beginning of the franchise and remains so. If you take X-Box out of the equation, the Marketplace is not required at all. Any developer can create aircraft that can be directly installed into the sim with a stand-alone installer and doing so does not require permission from MS or Asobo. 3rd party developers (like PMDG) do sell their products through the Marketplace as well so that they can be used on X-Box.
MS does take a “cut” of all Marketplace sales, but that has always been the case.
As far as I can see, the (potential) “encryption issue” only (possibly) affects the ability to modify default aircraft. At present, all default aircraft are streamed, so they cannot be modified in any case. We won’t know if encryption is going to be an issue until the ability to download default aircraft to local storage is implemented, and that hasn’t happened yet.
Even when downloading does become possible, some default aircraft may indeed be locked down, and should be. An example is the IniBuilds Airbuses. They were not developed by Asobo/MS, and although they are included at no charge to end-users as “default” aircraft in MSFS 2024, they are the intellectual property of IniBuilds, who may not want people to have the ability to modify them - and they would be well within their rights to require such restrictions.
A default aircraft that would be ripe for modification and improvement is the MAX, which was not developed by an outside contractor but is strictly an in-house Asobo project. Will that be encrypted? Until the ability to download to local storage is implemented, we don’t know.
Also, all the source code for the avionics frameworks developed by Working Title are published on GitHub for any developer to download and use/enhance/modify to their heart’s content.