I think you're right that lots of people use their software connected to a trusted Satoshi node that they run themselves, but realistically the wallet and GUI aren't just going to be deleted from the core Bitcoin code - people use them!
I wish for an isolation much higher than a more or less defined set of function signatures and structures within the same code base.
Such isolation was not a goal for Satoshi and is apparently not a priority for the core team.
I suggested such a fork (of code) to reduce the pain of code reviews and increase the pace of innovation.
It would come at a cost. Do you think it is feasible to branch and regularly rebase such a code subset for a "border router" ?