Every redesign introduces new (secret) watermarks, patterns that are hard to copy, so it's also a "tech" update. Eventually more and more of the details become public. By now the current banknotes are too well known and I guess the counterfeits are getting too close.
But if I wanted to create counterfeit euro bank notes I could just go with the old ones, no?
Anyway, I think creating new designs just for fun is a good enough reason for me to create new notes.
Maybe it is also just fun...
PS you can even vote for them! https://surveys.ecb.europa.eu/10b/neweuro/
An average lifetime of a banknote is 1-2 years for smaller amounts, 2-4 years for bigger ones. It is estimated that 15-25% of all banknotes are replaced every year. From about 30 billion individual banknotes in circulation that makes 4.5B to 7.5B each year.
So why not introduce new designs (and new protections?)
You can see all side-by-side here: https://www.ecb.europa.eu/euro/banknotes/future_banknotes/ht...
My takes:
Designs A and B and I are not terrible, but I'm not impressed.
Design C - the backside looks like the worst soulless corporate landing page graphics (Corporate Memphis style?).
Design D and E, H are okayish. Possibly I'd prefer something along these lines.
Design F has very strange coloring, the watercolor and sketch effect looks nice on screen, but I think more filled and more monochrome notes are more conventional and accepted, also some are bicolor. Not bad, but I'd not like it in everyday use probably.
Design G is somehow not good looking, I cannot phrase it. Both the font, and also the backsides scenes are meh, even the modernist buildings are better on some other series.
Design J : the creepy abstract cartoon people on the backside ruin it. Also somehow not aesthetically pleasing, but not catastrophic.