At the time Ariane 6 was getting started (publicly), SpaceX was still attempting to land Falcon 9 successfully for the first time. So it was not at all a sure b

At the time Ariane 6 was getting started (publicly), SpaceX was still attempting to land Falcon 9 successfully for the first time. So it was not at all a sure bet that Ariane 6 would end up coming of age in a world where individual rocket boosters have 10+ flights under their belt. Still, SpaceX was making progress at that time, so ArianeSpace pointedly ignoring the possibility of reuse was, in my opinion, shortsighted. Edit: SpaceX first booster landing was December 2015. Per wikipedia: Selection of the [Ariane 6] design concept was made by ESA in December 2014, favouring it over an alternative all-solid-fuel rocket option. Further design was completed in 2015 and it entered the detailed design phase in 2016. In 2017, the ESA set 16 July 2020 as the deadline for the first flight.