I attribute a big part of why content is burnt so fast to difficulty. Or rather the lack of any semblance of challenge.
It's the overall ease of content that means it gets consumed so fast. New patch lands you can have every piece of content in it cleared in a few hours. Most of it first time because there is no risk of failure. This is why a new patch doesn't even keep players busy for a single day.
You could then quite easily make content last longer just by increasing the difficulty. Now I'm not saying make it balls to the wall hard or anything like that but a definite step up from "lmaoroflstomp" where you can win without even knowing whats going on... hell theres dungeon bosses i still dont know the mechanics too even after running them 100 times. It just doesn't matter though because you can't possibly lose against the boss anyway.
That's why content is consumed so fast.
Now ultimately you can't really ramp up the difficulty without changing the reward structure because if you do you'll just end up with people not bothering like what happens with savage raids. Where people just don't bother with the content because the rewards just aren't worth it. And the challenge isn't worth it either because it'll be easy moded within months...
Reward out has to be worth effort put in. And ultimately the difficulty is so low because all the rewards are throw away trash...
It's not impossible to design content that lasts longer than a couple of hours and keeps players busy. And it wouldn't cost infinite amounts of moneys.. Just a change in design philosophy
The only way to do this would be to keep those dungeons out of the Duty finder or scrap the duty finder entirely..
I agree that big open dungeons with lots of choices of routes to take and bosses to fight and stuff would make dungeons much more fun than they presently are. but they wouldn't work in the duty finder.
All you need to do is look at POTD or HOH matched parties and you can already see the issues the duty finder causes there.
1 player in the group has no interest in chests just wants to unlock the portals and go go go
another player in the group wants to farm blue chests for aetherpool points
third player in the group wants to explore every single room looking for accursed horde sacks.
and who the hell knows what the 4th player wants to do....
and yet all of these players think the whole group should catar to them. that first guy will be standing on the portal saying come on come on come on. that second guy will have completely ignored that room full of mobs because there was no silver chest in it. but the third guy went in and pulled some mobs because he wants the horde sack at the back..
If you had dungeons that featured choices of routes and bosses and all that stuff it just wouldn't work in duty finder for the same reasons. you'd have one player that wants to go after Boss A because it has a drop he's after. another player though wants to go a different route towards boss B for a different loot drop. another player just wants to sprint through whichever route is the easiest / fastest.... and again everyone would expectthe entire party togo the way they wanted too...
if you scrapped the duty finder though this kind of dungeon content could work really well as you'd typically build a group of players who wanted to make the same choices and thus it would work much smoother. but then you'd have people complain they cant do the content without a fixed group. like how they complain they cant do hoh31+ in duty finder...
and thats basically why dungeon design is a s static as it is. because of duty finder.