The MEPHISTO is an end of line vehicle in the tech tree. End of line vehicles have always had higher module RP costs (that decrease if vehicles are added after them) so this isn’t anything new and this also applies to all tech trees so it’s not something exclusive to France.
End of the line RP costs are stupid because eventually they’ll add more stuff and you’re again forced to grind end of the line RP and again, again and again.
Another system, like practically every system in the game that needs to be reworked, you shouldn’t be punished for researching a vehicle earlier than someone else.