I think that's what makes UEL interesting. I mean if the big clubs that were knocked out of UCL then won UEL I think it would make UEL less interesting because it looks like a weaker competition because the champions are those who were already eliminated from UCL. But the fact is UEL is interesting to watch when "normal" teams are able to beat big teams from UCL. On the other hand UEL gives a chance for European clubs that are not in the top European clubs to play in international competition, compete with top European clubs and get attention from fans or talent scouts.
What you said makes sense but for this season it is no longer the case because those who are eliminated from the UCL will still return to their respective domestic leagues, as well as the UEL where teams that fail to enter the playoffs will automatically think about their fate in their own domestic league. So for this season the level of fierceness of the matches in the UCL and also in the UEL is very different so that both are still quite interesting to follow and watch for many people because in the UEL there will also be many teams that will fight to be champions and get a place in the UCL.