So that‘s a severe political problem for the other member states who might want to go to an all-liquid propulsion form for the Ariane 6. But there may be a way to get there anyway. If a member state wanted to spend their own money to build a prototype Ariane 6 core using two Vulcains how could other member states prevent it? It’s their own money. They can spend it anyway they want. Remember quite key to why this is approach is preferable is because how low cost the development costs would be. The example of JAXA adding a second hydrolox engine to the H-II core for ca. $200 million(27 billion Yen) demonstrates this:
In point of fact it’s probably even cheaper than this just to add the second engine. The transition from the H-IIa to the H-IIb actually involved multiple systems:
Then conceivably the development cost just for adding the engine only might be only $100 million or less. But when there is no multi-billion dollar development cost, any of the ESA member states could afford to add an additional engine to an Ariane 5/6 core on their own. It’s so low that even the member states that already spent billions developing the solids could also adapt a Ariane core to have two Vulcains at this low cost.
At such a low development cost and each per rocket cost being even lower than the Falcon 9 each ESA member state could have their own independent all-liquid Ariane launchers. And each ESA member state could have their own independent manned flight capable rockets.