Principles are ways of successfully dealing with reality to get what you want out of life.
Ray Dalio, one of the world’s most successful investors and entrepreneurs, cites principles as his key to success.
A "theoretical should" occurs when people assume that others or themselves should be able to do something when they don't actually know whether they can (as in "Sally should be able to do X, Y, Z"). Remember that to really accomplish things you need believable Responsible Parties who have a track record of success in the relevant area.
A similar problem occurs when people discuss how to solve a problem by saying something vague and depersonalized like "We should do X, Y, Z." It is important to identify who these people are by name rather than with a vague "we," and to recognize that it is their responsibility to determine what should be done.
It is especially pointless for a group of people who are not responsible to say things like "We should . . ." to each other. Instead, those people should be speaking to the Responsible Party about what should be done.