Mistwell
Crusty Old Meatwad
I also find it interesting that you are defining degrees of similarity and using that to make your "ruling" when the rules never mention objects similar in some ways but not in others.
What rule allows you to say shields are similar enough to maces to be treated like a mace for damage but not similar enough to a mace to be able to benefit from proficiency?
The second paragraph spells out the first two levels. You treat it as the weapon damage if it's "similar". And then "At the DM's option, a character proficient with a weapon can use a similar object [at proficiency bonus]".
To me, the DMs option is saying "DM decides - is it so similar that it's 'as if it were that weapon' (the language of the rule)?" If yes, may use proficiency. If on the other hand it's not that high level of similar but still similar, than don't use proficiency. Regardless, use the weapon damage die for all levels of similarity.
Third paragraph is the third category - "An object that bears no resemblance".
5e uses natural language in the rules, and intentional DM flexibility for judgement calls in those rules. The natural language of "bears no resemblance" is to me "it's nothing at all like". And the natural language of "as if it were that weapon" is to me "it's almost the same." And, obviously there is something between "it's nothing at all like" and "it's almost the same" and the rules offer a suggestion on how to rule that middle ground - with a DM's call on whether or not you can use your proficiency bonus.
That's how I read this rule. And I don't think it's at all a stretch (in fact I have no dog in this fight - this hasn't come up at my table that I can recall). This is how I read a lot of 5e rules, and based on reactions I see here and at reddit and on how the authors explained their thinking on various rules like stealth, I think this is how the authors intended rules like this to be used...with a flexible range of how to handle things and a lot of DM judgement calls as to where something falls withing that range. It's pretty rare that the rules give you only an on or off resolution. Where it can be done, the authors usually provided more than just those two types of rulings for the DM to make. And that's what I think they did with this set of rules.