Re: Oh, _that_ rabbit hole
I think the explanation is a little more complex than just third person usage. Instinctively, as someone who has been using English for more decades than I care to think about* the axis appears to be about more than singular vs plural. It's also informal vs formal, intimate vs impersonal and definite vs indefiinite.
Referring to oneself the usual pronoun is "I" but formally it can be "we". Hence the "royal we" for proclamations although it can be used in non-royal legal usage. It can even be used in less grand situations than that: habitually on cooking programmes a chef will explain what "we" are going to do although maybe that may be a case of not adapting to working solo instead of with a team. It's also not unknown for someone caring for a sick child to explain that "we" have not been feeling very well.
As regards 2nd person the rules for thou/you were (still are if you want to use them) exactly the same as tu/vous in French. The Yorkshire rule as said by a senior to a junior is "I can thou thee but don't thee thou me". I'm not a linguist but I gather German is even more complex.
As to third person I can't better the example someone gave on an earlier thread: "See who's at the door and find out what they want.". I agree this new usage can be a bit jarring but on the other hand, as a male, it's good to have my pronouns back: females had she, etc. to themselves but we blokes had to share our gendered pronouns with the general case.
English has cut down the complexity it seems to have inherited from its Indo-European roots but don't let's lose all the subtlety.
* and was brought up in a time and place where the 2nd person singular was in use.