Hello MonsieurNebo,
Something to consider when deciding what to do with these contacts is to look at the industry regulations which apply to your business. Are you based in the EU do you need to follow GDPR regulations, are your customers based in the EU? etc.
I agree with you data should only be kept if you have concrete justifications for why you keep data for the periods you do. Does your business have a data retention program?
Even if your not based in the EU you might some of this guidance useful - Principle (e): Storage limitation | ICO
Have you thought about creating a subscriber re-engagement campaign then removing contacts that Bounce or Don’t opt-in.
All the best,
Jaimee