Offering "local" IBAN would basically require bunq to set up local operations / subsidiaries and getting regulatory approval in every country supported. A bank to set up local operations and spend immense ressources on that - I think that it whole unwarranted. It also wouldn't "solve" the issue at its core.
It can be summed up quite simply:
Foreign IBAN are supposed to be usable in ANY country (within SEPA, that is) without discrimination.
That's the law (though neither perfectly defined, phrased nor implemented).
Companies discriminating against foreign are plain and simple violating the law (in most cases)
Your suggestion would neither address the problem at its root, rather than merely mitigating or circumventing it. It might actually encourage companies to keep violating the law. And spending immense ressource on that seems just plain wrong to me. It's like giving in to and tolerating violation of the law.
The French need to get their act together. If companies don't give in, complain to the national authority designated to take enforcement measures against IBAN discriminators. If they don't get their act together, complain to the European Commission.
Don't spend any money on local IBANs cause some stubborn companies refuse to follow the law!
I'd rather see my bank spend it on functionality and innovation.