I understand that, but a reusable engine is not the same as a reusable rocket. If we are going to discuss engines, it seems that the appropriate comparison is the development of the Merlin-series engines, which started around 2004.

> Arianespace moves slow, but they constantly move forward.

... way more slowly than SpaceX. That's not how a company could survive in the free market.

Wikipedia: > The European Space Agency began funding the Prometheus development in June 2017 with €85 million provided through an ESA capital mechanism called the Future Launchers Preparatory Program.[1]

Based on what information have they been spending 'years' on that engine?