Yes, that's what "standby" tickets basically are. Some airlines would (not sure it's still a thing) sell you a "standby" ticket for cheaper than a normal ticket, which meant you show up and wait at the gate until near departure time, and then if there are any seats available they let you on the plane. If not you have to wait for the next flight.
Yeah, but the problem is that most tickets are now, in effect, standby, but not labeled as such. Hence why the parent suggested making the status (and the option to upgrade) clearer and more readily available.
On Delta the Basic Economy (E) basically are standby since you can't select your seat and are only assigned one at check in (or at gate? can't remember). It's not even that much cheaper than regular main cabin.
According to the article, 4% of planes are overbooked. And if they are, <5 will usually be re-booked. Which is a low single-digit percentage of passengers. So your chance of being denied boarding is probably <0.1%. Wouldn't say that all tickets are standbye.
It's rather that thanks to the internet we know of stories like these. 10 years ago no one would've known.
I say that you're "effectively on standby" when there's a potential for the overbooking to keep you off the plane, which is the kind of risk that should be made explicit.
