This feels a bit redundant. If you were in a country taken over by dictators or terrorists, those dictators or terrorists won't hesitate to deploy a mandatory surveillance regime (see: Xinjiang), they don't need a leg up from the previous democratic government. If your country is not overtaken by dictators or terrorists, the "nothing to hide" argument stands.
This hypothetical situation is not made materially worse by the existing backdoor.
No, it doesn't, because it's not an argument. It's just an assertion--and one which, as another poster has pointed out, is false. Everyone has things to hide. But "hide" is not a pejorative term here; it's just a recognition of the fact that people have private lives that they don't want to share with everyone. The so-called nothing to hide "argument" depends on ignoring that fact and treating "hide" as if it had to be something nefarious.
Setting up a backdoor is more difficult than co-opting one that already exists. It's called the bootstrapping problem, and you posting this right now are the beneficiary of a hell of a lot of bootstrapping.
So your assertion fails before you even get to merits.
I don't understand what you mean by "bootstrapping" in this context. If you can get your phone or computer randomly inspected and copied on the street, and then thrown in jail or executed for trying to avoid surveillance, there is no need for "backdoors" or "bootstrapping". The government just passes a relevant law and surveils, no need for fancy backdoors when you can do straight interception and bans.
Besides, "you need to protest RIGHT NOW so that you can delay total surveillance regime by a year or two if the government eventually turns North Korean" does not sound like a strong call to action to me.
This hypothetical situation is not made materially worse by the existing backdoor.