> But from passengers' point of view, there aren't always enough people who'll voluntarily accept an offer to transfer to a different flight once they are at the gate awaiting boarding

There are; they just won't accept any offer. Most of the airline offers I have been offered in this situation were paltry: reschedule and some airline "credit".

Give me a partial or full refund, and I'll be much more willing to volunteer. Otherwise, you're seeing passengers value their time higher than some credit and lost time.