There are times you need speaker-phone mode. My parents almost always turn on speaker-phone when they call me because they both want to be part of the conversation. I don't think they will ever take a plane or a bus trip in their lives so their speaker-phone isn't going to hurt anyone.

I need speakerphone when I'm home alone and attempting to be on a phone call while doing other things. Some of those calls are even about instructing me to look for something, so it necessitates me to be moving about. Speakerphone is an incredibly useful utility.

Don't take functionality away because of a few bad actors. That'd be like getting rid of drones because a few people are assholes.

Put rules in place to correct the bad behavior. Kicking them off planes seems fair.

Speaker phone is probably necessary nowadays since many people only have a smartphone.

With the old phones you could reasonably tilt your head and raise your shoulder to hold the handset in place so you could do something that required two hands while talking/listening, like looking up something in a book or taking notes.

Smartphones are smaller than the old handsets and much flatter. I can pinch mine between my shoulder and head but I've got to raise my should pretty high and do some other contortions to get my head tilted enough making it much more awkward to do anything with my hands. Also that phone is small enough that it is pretty well covered in that position by the side of my face and my shoulder, so I'm not sure the mic could pick up much.