First steps to a closed platform ??
Make a IX could be possible with mods but there are some problems, for example the external model, different length of the submarine interior, stern torpedo tubes, damage screen, crew, ...
And this assuming that ubi donīt want to sell aditional submarines in addons,
this can limited a lot the mods for the game.
DLC is a way to sell a incomplete game to the public and do business with the missing parts