Oh, I think this is very difficult. First of all because it is not a single reason why a future where privacy has eroded is a very bad thing, but rather many different reasons. This makes it difficult to know where to start, as it will depend on the person you are talking to what they are more receptive to. Concepts such as security, privacy, secrecy and anonymity are often confused. You have different actors you would want protection from, including corporate and governmental entities.

I don't think most ordinary people you meet will be bad faith actors though, but I do think many tend to take offense if you are outspoken against something that is proclaimed to be about protecting children. Why wouldn't you want to save children?

Some of the reasons below, but not an exhaustive list. As I said, difficult to know where to start.

1. You do have something to hide, even though you might not be doing anything illegal (to your knowledge). Most people dislike people staring into their living room from the street, and will install curtains or other ways to prevent it. Most people closes and locks the door when they go to the toilet. Most people do not say every single thought they have out loud. I think the disconnect comes from people not actually knowing what data is collected, and even if they do, they do not understand how this data can be used / misused to learn things about you or manipulate you, and the privacy threat of having this data stored anywhere even if it is not being used (i.e. risk of data leaks). In terms of manipulation, I think that the story on how Facebook nudged people to vote in the Scottish referendum highlights the creepy influence such a company can have on society, and this was already in 2014. Who's to say the owners of such platforms will not use it to sway elections their preferred way by using such nudging tactics on the population they want to vote, and not on the ones they'd rather stay home. We shouldn't have to trust that they don't abuse such a power.

2. What today might be perfectly legal, might not be legal tomorrow. Case in point are the draconian abortion laws implemented in various states of the US. Facebook had to comply with government requests to hand over chat logs..

3. What today is illegal, should perhaps not be illegal. We do not want 100% law enforcement, as that would mean that we consider today's laws final, however we are constantly evolving our laws. A recent example is legalization of weed in the US. How many have been incarcerated and had their lives ruined on charges related to weed? Yes now the same activities are in many states considered legal. Or homosexuality? Sodomy laws are not a very distant past in many countries (and still exist in other places of the world). If you had Apple or Google scanning your phones and flagging you to law enforcement for illegal activities. Effective mass governmental surveillance (and corporate surveillance that can be passed on to law enforcement) could potentially send countless people to jail on charges that could be legalized in just a few years.

4. Building an infrastructure for mass surveillance is not future-proof. You might trust your government not to misuse it today, but what about after next election? There are countless examples of less-than-democratic forces gaining power in Western democracies in recent years. We need strong protections against potential oppression/suppression, and not just soft protections that are easily swept aside.

5. We are dependent on journalists and whistleblowers exposing wrongdoing in our society. Lack of tools that ensure privacy and anonymity prevents this.

6. Even if our societies are not oppressive regimes today, many around the world are. Political opponents and resistance groups in such regimes need ways to protect themselves. Otherwise authoritarianism will have an too easy time to crack down on dissidents, making organized opposition impossible.