As recently as Oct and Dec, I have requested credit once onboard for something I prepaid prior to embarking and had ZERO issues. Simply let them give me OBC (it automatically posts as Refundable OBC) and at the end of the trip either have it refunded to CC or cash it out. This has worked for spa, excursions and dining. All requests need to be made with the appropriate desk (ie excursions at Excursion desk...) not at Guest Services. I did this on Nieuw. Amsterdam, Nieuw Statendam and Koningsdam, worked flawlessly.
Have also made Pinnacle reservations using HIA by simply calling rather than doing it online. They applied to HIA credit and it shows as "redeemed" in the booking. Did that last summer (2022) for Dec 2022 & Feb 2023 cruises without issue.
Agree passengers simply trying to use what they've paid for earned shouldn't have to jump though hoops or be disappointed, but sometimes we just have to find a way to make it all work. I'm not looking to make a vacation a stressful ordeal...
And agree that plans/schedules/things change and there needs to room to accommodate those changes so capacity control should certainly be in play. Especially as ships are once again at capacity.