Would we be better off today if those in power then had been able to silence unpopular opinions?

They definitely could, it just didn't matter because people spoke up anyway.

First, every individual has what should be a rather obvious and fundamental right to think whatever thoughts and have whatever opinions they like. Nobody gets to decide what opinions an individual holds, except for that individual.

Now, as for the spreading of those ideas:

Power to censor is frequently used to silence good ideas and allow bad ones to flourish - see modern-day censorship in China, Russia, North Korea, India, and Australia, or older censorship around the Vietnam War and slave liberation (back when owning slaves was in fashion).

The fact that someone is silencing an opinion has very little to do with whether or not that opinion is actively harmful - and, at the very least, those in power have a strong tendency to use their power (including censorship, if available) to gain more power, and those in the public realm will use their own speech and censorship (including via "canceling" aka mob justice) to benefit their own group, regardless of whether their cause is just or their tactics are fair.

So, it's all well and good for you to say "this opinion is harmful", but that's not really well correlated to whether that opinion is actually harmful. Even when the majority of the country agrees that an opinion is harmful, that doesn't actually mean that it is - but, as a democracy, we choose an imperfect system that works most of the time. That system, however, involves passing laws that declare some speech harmful, based on representatives elected by majority vote of their constituents. Twitter-based outrage from a tiny fraction of the population defining "bad opinions" is so antithetical to the idea of democracy that it shouldn't even be considered in the first place.

tl;dr we have an existing mechanism for controlling "harmful speech" and it's the government, not social media mobs.

Private dialogue among friends is just that.

Unless they are actively plotting violence or some kind of action, or making unambiguous bad statements such as 'I hate people of this group' or an admission of a crime, then otherwise, it just shouldn't be news.