The thing is that the moment 1 country voluntarily decides to mandate it, the infrastructure for the backdoor needs to be set up in place, as a mandatory requirement.

For it to be truly optional there should be a demand for the opposite too (ie. countries placing fines to those who do chat scanning), otherwise it doesn’t make much difference… any chat app that wants to work globally in the EU is gonna end up implementing the scanning EU-wide, even if there are countries that don’t enforce it.