You still need to do backups, a database backup is just one part of that, if you are not following the 3-2-1 rules and don't test your restore mechanism, you don't have reliable backups.
Those things you listed are sill sysadmin tasks in my eyes, and you are doing them, validating my point.
You still have to track storage space, either because you are paying for it and need to expand when necessary, or you have to manage costs at one point, that's not completely out of the picture. It can be easier for sure than building your own storage hardware.
You still need to keep systems up-to-date either you are using Docker so you are doing it on your "application level" or you are using Linux VMs and you need to upgrade those systems/images. Even if you are using something like Functions or Lambda, those have their own environment which you need to be aware of and they usually support specific versions of programming languages, so you need to upgrade your own stack when they don't support older versions anymore.
I tell you that ECS has eliminated a ton of extra work for my team for a bargain price, and your response is "but you still have to do x, y, and z!" It's like saying that I shouldn't buy a dishwasher because I'd still have to wash the pots.
Yes, we still need to do some sysadmin-y tasks. But ECS handles so many of them that we actually have the time, energy, and knowledge to take care of the few that remain.
(As an aside, keeping language and OS versions up to date becomes a development task rather than an ops task when running Docker + ECS. We increment a version number in the repository and test everything, the same as we do for any library or framework that we depend on.)
Those things you listed are sill sysadmin tasks in my eyes, and you are doing them, validating my point.
You still have to track storage space, either because you are paying for it and need to expand when necessary, or you have to manage costs at one point, that's not completely out of the picture. It can be easier for sure than building your own storage hardware.
You still need to keep systems up-to-date either you are using Docker so you are doing it on your "application level" or you are using Linux VMs and you need to upgrade those systems/images. Even if you are using something like Functions or Lambda, those have their own environment which you need to be aware of and they usually support specific versions of programming languages, so you need to upgrade your own stack when they don't support older versions anymore.