Also, vandalism is about the least interesting reason to hack kiosks. It can get you into an otherwise inaccessible network, which often contains all sorts of internal services with loose or no authentication (POS software often uses default creds because "it's on an internal netwok anyways"). Hacked kiosks are also often used as proxy servers for illegal activity and bots in DDoS botnets.
The update process can be backed on the kiosk side, hacking the remote side of the update process is a completely different story.
I mean in some cases that was a simple signed package hosted on an S3 bucket... how are you going to leverage that to vandalize a network of devices?
And the kiosks are never on an interesting network (if they were there's dozens of ethernet ports scattered about the place you can use to get access anyways)
Hacked kiosks being used as proxy servers when you need physical access to hack is also a very uninteresting problem. Why risk tying your physical self to a bot for nefarious usage when there are a million and one other "IoT" devices you can pwn instead?
If you want to get creative with attacks you can, but sometimes comparing a creative attack to a "boring" attack can help frame the conversation.
I've written kiosk apps that landed across the US and while there was a ton of hand wringing about security, and in an informal setting I brought up a simple question:
If you reverse engineer the update process to have it show a penis, or you just carve a penis into the public display with a pocket knife, what's the difference and which is more likely to happen?