Having to walk a guest when the hotel cannot meet the terms of the reservation is part of the contract between the big hotel brands and franchisors. If the hotel operators do not fulfill the walk policy, then you should contact the 800 number for Marriott/Hilton/IHG/Choice/Wyndham/etc and they will mail you a check for your expenses and bill the hotel owner.
This thread started about “boutique hotels” and now we’re talking about franchisees of major national chains. They’re not the same. Plenty of tiny unfranchised operations can you leave you hanging with little to no recourse.
I worked a few years as a night manager in a hotel in Norway - we always had to figure out a solution when overbooked - at "our" expense (customer pays/pre-paid agreed upon price, we cover room and transport at alternative accommodation - usually more expensive).
As a bonus re: booking.com - they integrated with our booking system and always overbooked, unless our system lied about available rooms (would happily sell rooms when none were available - especially annoying at night, when a booked room typically means an occupied room).
Or maybe booking.com is actually a better experience for most people than the legacy hotel sites. It's more streamlined, it's cheaper, you don't have to sign up etc.
If you think I'm an astroturfer look at my comment history, I've never mentioned hotels before.