The arguments supporting Plymouth are simple, and I believe there are just two of them.
- Plymouth would not need any new cars.
- Having Plymouth would allow the much-needed brand focus for Chrysler and Dodge.
To counter your arguments, Plymouth is seen rather favorably by most Americans, though they are vague on what it is, and it's still relatively recent. Bringing back DeSoto or Packard would be hard. Plymouth, not so much.
To bring back Lancia, you would need to create new Lancias. You would need to differentiate Lancia strongly from Alfa Romeo and Maserati somehow. Where would Lancia fit exactly? With Plymouth, the primary costs would be marketing, legal, and dealership signs (paid for by dealerships!). All the vehicles exist, it's a matter of moving them from Dodge and retargeting future cars.