[structural]

First, because taking aircraft out of service for design issues is incredibly rare (less than once a decade across all kinds of passenger aircraft).

Second, because airplane manufacturers have a significant amount of negotiating power, combined with airlines very often not being in a great financial position: Boeing certainly wouldn't want to be liable for storage costs and the logistics of getting a fleet of planes back to a central facility if an airline went bankrupt, for example. Leases also get legally interesting when the assets involved move internationally on a daily basis...

[SilasX]

I wasn't proposing that any of that and I'm not sure it's relevant (e.g. in this case it's a software update that can be done without moving the plane); I agree they wouldn't want to take over a dead airline, and such an agreement doesn't get the incentives right. But it's common for OEMs to take the hit for things that are their own fault, if it isn't legally required.

And the rarity of the incidents plus Boeing's asymmetric knowledge of them would favor Boeing being a guarantor. And remember, I just said the design issues. Obviously the airline would be expected to take hits from e.g. FAA groundings from their own maintenance failures.

Edit: That leaves asymmetric bargaining power in your reply, but I don't see OEM aircraft competition as being so monopolized that they wouldn't compete on "hey this company makes us bear the costs of their design problems but this one doesn't".