Ultimately I am the one paying the bill currently so if I die nobody elses credit card is being charged.
In terms of other admins, this is actually happening. Some smaller instances like mine are in the process of setting up a sharing admin work between instances so that if someone is on holiday, the instance still has an admin who can login. This was only just started and is in the process. We have to create a lot of documentation and basic stuff to get it fully functional where another admin can login and fix something. Its not at that stage yet and will be a couple more weeks before it is. We did a test last night where another instance admin (boulder.ly) could connect to my instance via ssh but without documentation on what to do and check anything more than the basics of rebooting or restarting something isnt going to happen. Eventually we will get it to what to do if site has a critical vulnerability or is being attacked but not ready yet. Its a work in progress unfortunately.
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Yes obviously the barrier to entry is high. But nobody knows for the big servers either since they are basically just small instances that happened to get big. Thats why lemmy.fmhy.ml just died one day due to domain seizure. End of the day all you can do is look at how long a server has been around and if it has be online a reasonable amount of time. That kind of reputation just increases slowly and nobody can make it happen faster.