I have read several similar questions which provide partial answers but leave me with some uncertainty. I am confident I understand that the answers say the probability and severity of damage to the power supply is high. I have found a variety of questions and answers which are specific in regards to damage, but they are about USB devices attached to computers, or provide a definitive answer about devices like motors etc. I have also found a lot of good information about the design, application, safety of operation, and the selection and performance of power supplies. But I have found that there is a common thread in various questions that I think is implied by the OP, but which I felt wasn't directly addressed, and that is one of concern. Their concern seems to be, and mine definitely is, from the perspective of a consumer in the context of having made an investment in the purchase price (it's all about my shiny new gizmo!) The question isn't one of technical details, as much as it's one of the ownership of personal property (what kind of risk am I taking where I'd wind up throwing it in the garbage?) So what would you tell someone who has no technical knowledge of electronics? Or how would you educate a casual hobbyist to give a confident answer when someone else comes to them as the expert? "I don't care about sacrificing the adaptor, but what's the chance I'm taking to mess up my Raspberry Pi by using a power adapter with the correct voltage but not enough current?" (Taking a hit on performance won't matter if buying the correct power-supply will restore it to full working condition.)
"I don't care about sacrificing the adaptor, but what's the chance I'm taking to mess up my Raspberry Pi by using a power adapter with the correct voltage but not enough current?"
Unless the Raspberry Pi is extremely poorly designed (which I very much doubt is the case) or unless the power adapter does not limit it's output to 5V, you will not damage the Raspberry Pi. (It may not run, but that is not your question.)
@Justme points out below that if the power adapter is damaged by being overloaded, it is conceivable that that the damage would be such that it produces a voltage in excess of 5V. I guess there are no 100% guarantees in this life.
So what would you tell someone who has no technical knowledge of electronics? Or how would you educate a casual hobbyist to give a confident answer when someone else comes to them as the expert?
I would tell them, "Don't be penny wise, and pound foolish." Buy a power supply that meets the specs. Using a power supply that cannot supply enough current will probably not damage your RPi, but it will almost certainly create problems that will waste your time and work against your objectives for purchasing this device in the first place.
I am running several PIs from nonUSB power adapters and one is for sure - if normally Rpi takes whatever current, your power supply should supply twice of that. On my Pi4 when i connect external rotating hard drive, my screen will go off when the platters spin, then it clicks and screen goes on. But Rpi runs normally (it suggest my power adapter cannot handle enough Amps to it) but it is never damaged and runs ok. Even with lower voltage, Rpi will run but show a thunderbolt sign at the top right of screen. I did many trial-errors with power delivery system through 5V GPIO rails and never damaged it