As a german I’ve never heard about iDEAL. The Wikipedia article says that it is mainly used in the netherlands

The more literal soundtrack

That already exists. systemd-run is already available today. So the attack surface would be smaller

I think the article (or more Lennart Poertting post) explains it quite nicely. The problem with sudo is that the sudo binary itself has the ability to gane elevated privileges which is a potential attack surface