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.