Correct me if I'm wrong, but if you own > 50% and get the ability to reverse transactions, it can still go undetected. So it's possible to hurt others without hurting yourself.

How would it go undetected? The people whose transactions are being reversed would definitely notice, as would anyone else paying close attention to said transactions.

In theory yes, but in practice the people most involved (holding large stakes) would pull out of the system en masse, crashing the value and (probably) the whole network with it. Same with BTC. In theory anyway.

What if those people had more to lose by pulling out than pretending nothing happened as long as the 51% attacker does not abuse it "too much"?

It definitely doesn't go undetected, as the conflicting transactions would have to be in blocks both forking from a common block, both signed by the attacker, which is a slashing condition that would burn their staked ether.

Also, it would be kind of visible and hard to hide. With sufficient high profile stake holders, this would be very unlikely to happen and even harder to hide.

Ok, but suppose someone owns 50% of the tokens. Doesn't this mean the system is de facto centralized. Even if the majority holder behave well, how is it better than a centralized solution?

Unrealistic surely but, "Some just wants to watch the world burn"

Another aspect of PoS: it makes these attacks incredibly expensive. Currently staked value is over half a billion USD.

Someone (very rich) could bet even more money on that thing falling..?

If a large government decides it's a problem, would they hold it or destroy it?