Yes, but how ethical is it to ask for volunteers at half the compensation that they would be required to pay for an involuntary denied boarding?
Depends entirely on the individual passengers circumstances and what their ongoing arrangements might be.
If changing flights didnt do any more than delay my arrival at final destination by an hour I would happlily accept 50 quid. If one misses ongoing connections then the amount required might be very large.
Asking for volunteers to disembark is a perfectly reasonable arrangement, which, in my limited experience, works very effectively.