99% of consumers don't care, especially when there's a 3rd party to be blamed. Now that it doesn't look good for Marriott what are you going to do? This is the Coca-Cola corporation vs Pepsi Co problem. You can choose not to drink coke and Pepsi, but if you want to cut of ties with both the mega companies in this field you're going to start having problems, it's not like you can drink Sprite, or Mountain Dew, or even water in some cases.
Those people who booked through Marriott into Sonder properties were covered. I don't know why you expect Marriott to cover some 3rd party business's bankruptcy losses.
Also are you sure Hilton contracted a 3rd party for a convention? That sounds unplausible. Normally the convention holders contract a 3rd party because they think Hilton charges too much convention / bulk booking services (which they definitely do), unless you were attending a hospitality convention, then what you said would make sense. You have a beef, but I doesn't look like it is with Hilton.
By the way there's countless Hotel customers who are one-offs in the world. It's rare to find repeated customers and the way hotels do that is with loyalty programs. I've only got some 30 nights with Hilton myself, but there's been plenty of times with another chain I stay at a LOT where they just don't actually give a crap about their policies anymore. It's hard for any company to know whether they are trading 1 night or 100, and the major chains are just big enough that they don't need to give a shit about individual customers anymore.