I worked on the Ariane 5 during the transition from Ariane 4 to 5. Back then many did not like the Ariane 5 design. The Ariane 5 was originally meant as a carrier for the European Space Shuttle competitor Hermes and repurposing it for commercial payload launches involved many compromises. When the last remaining Ariane 4 had launched I asked one of my senior colleagues why we wouldn't just build a couple more of them. I was told that this was not realistic because for one, parts were built in batches and you could not just produce a few and for another many of the companies that were involved did not even exist anymore.
Regardless if that is true, the last Ariane 4 launched 20 years ago so it is certainly a rocket of a long gone past.