And a +1 sword is only boring in certain contexts. Believe me when I say that a +1 Shortsword can sometimes get a table grinning from ear to ear.
Not everybody is an optimizer, a powergamer or has the system mastery to catch if a folding boat is more game changing than a +1 sword of boringness. The point of giving players free reign over magic items is so they can have fun with them, not to promote powergaming.
The problem is that 3e let the cat out of the box.
And a +1 sword is only boring in certain contexts. Believe me when I say that a +1 Shortsword can sometimes get a table grinning from ear to ear.
I guess it all comes to individual styles, for me a +1 sword is always boring, the only math item I could find interesting would be an uncursed -5 sword.
Okay, so we agree that sovereign glue is a terrible example due to its obvious poorness?
I'm a natural powergamer (I "see" numbers quite easily) and my observation for you is this: 5E is so easy (per official encounter tables) that if you stick to encounter guidelines, players can do anything they want and come out okay. I have no idea if a folding boat will turn out to be worth anything to the players or not, but a +1 sword of boringness isn't worth all that much either, in terms of expanding the scope of play. (Yes, it will double the fighter's damage against elementals, but if he was going to win that fight anyway, all it really does is save him a short rest.) Let's say you allow PCs to purchase magic items freely from some wizard shop, at DMG prices. What's the worst that will happen? Maybe some guys will buy super-effective stuff like Wands of Web and Winged Boots, while others buy Leather Armor +2 (which, honestly, is terrible). That essentially makes it like the guy who bought Leather Armor +2 threw his money away--and it doesn't matter! He will still be an effective character against anything in the MM that follows encounter guidelines, because 5E is easy by design.
Now, if you're the kind of DM who likes to throw your PCs into the deep end ("you're now being chased by twelve Fire Giants; what do you do?"--note that that would be a Deadly encounter even for a group of 11(!) 20th level PCs, but I would totally do it to my current group of 3rd through 7th level PCs) then maybe optimization becomes more important. Even then, why not just have the guys who are good optimizers give advice to the guys who aren't?
In short, there's nothing in the game requiring you as DM to rejigger the DMG prices for magical items before opening a magic shop, unless you want to. This is in spite of the fact that the DMG prices are almost uncorrelated with utility.
I'm in the opposite side, I just barely understand optimization from 3.x let alone another edition, I picked the most obvious one because it is the most obvious one, there have to be some other ones that are invisible to me, but that is worse, all the experience i had from previous editions is useless. And that's why I need a list based upon utility because I don't really know which is more unbalancing. (And because I suck at using the encounter guidelines I go for what makes sense at the time instead)