The best European champions rapidly make the transition to world level so those with the longest reigns as European champion tend to be among the weakest.
Although with hindsight, I was mighty naive (that perhaps being that I was barely a teenager), but I was very impressed with long term EBU 175lbs champ Alex Blanchard, and a draw with Dennis Andries showed he was not without ability. In the same era I l also liked Gert Bo Jacobsen, Anatoly Alexandrov nearly always impressed me. Patrizio Oliva had a bit of class.
I kind of agree.... But a lot of them made good money fighting as European Champion, so did not really need to step up.
Gorgeous Georges Carpentier was indeed allegedly French Champ at least Lightweight though to Heavyweight (some say at all the classic eight divisions); and according to the brilliant BBB of C Yearbook (2009; Edited by Barry J Hugman) was European Champ at 147 (1911-12); 160 (1912-18); 175 (1913-22) and Heavyweight (1913-22).
But Carpentier was a world Champion at least 175, as well as claims at 160 and Heavy. I think the idea of this thread (I may be wrong) is the best Euro Champ, with no World Claim, because as you rightly pointed out, if you were good enough, you made the next step.....