... and Aerosoft is happy to sell the products, as this is the money they are pay from at the end of the Month.
The developer, as long as he has no sponsor (Papa,Mama or so), must pay his bills too. This will be impossible, if you work weeks to make the product compatible for a new plattform, optimize the code for the needs of the new simulator engine, add new feature and so on and everybody gets it as a gift for free. We don't talk about bugfixing here, as long as the P3D V4 is not defined as a full repayed Bugfix of V3
Software is not building it self, there are humans behind, which spend time and knowhow into it, this needs a refund (payment) as every person request for the job they do.
Everybody can use the addons he has payed for on that plattform they was developted and selled. When he change the plattform (by paying the full price for in many parts the same code btw) must expect, that addons needs invest to follow the changes and need to be updated for the new parts with a small fee.