As one of those completionist kids, Let me give you a couple examples as of why this is boring & bad game design.
The complaints of "things being tedious & boring" have always been there. People complaint about it when the first 9 tall tales came out & for the gold curse you had to do all 9 tall tales 5 times! Every person said the same thing "I am never doing something so tedious and repetitive ever again".
Then another time there was an event for a gold hoarder figure head were you had to catch 300 small rubby splashtails, and again people didnt like it at all.
Why is that?
It is because it is not challeging, engaging or fun at all, rare just slaps a number behind it and calls it a day and it just becomes tiresome to grind since you're doing to same over and over.
They now introduce concepts like sleeping & sitting wich isnt gameplay and again slap huge numbers behind that. You cant tell me that sounds like fun gameplay do you?
It is a game, games are supposed to be fun & possible challeging while you're doing anything.
let me take world of warcraft as an example here.
you had the title The Immortal. You could obtain it by venturing into the 25 man naxxramas raid, without a single member dying at all untill the final boss was defeated.
That takes time, its fun & challeging.
Later on they introduced achievements like glory of the "Blank" Raider. (since this achievement applied to multiple raids).
You had to defeat every raid boss but there was a twist. You had to defeat every boss in a special way.
The raid boss Lord Marrowgar from the raid Icecrown citadel does an ability were he spawns in ice spikes, the twist was, nobody was allowed to get hit by it for you to get the first achievement he gives you, or you had to retry next week.
Every raid boss had some twist behind it, and if you defeated them all while getting all these achievements on every boss, you got a special mount to show it off.
Warlocks during mists of pandaria got a special storyline to get a cosmetic piece that turns their fire spells green, it was a whole storyline were in the end you have to defeat a boss that was pretty tough. And you only had to do it once.
Some of these example are definetly bit overboard.
But the picture is, Even tough it takes time to get these, it was fun, not tedious or repetitive.
It was an anticipation of "Wil we finally get it this time?".
The point is, they came up with creative ways that actualy have stories behind them, and wen you saw someone with a mount or title from one of those achievements, it was actualy impressive.
Nobody is going to be impressed or have fun doing this X amounts of times by sitting or sleeping or even catching 2500 specific fish.
That is the point you dont get.
Not everyone is a completionist like you make these "kids" out to be, People want to do them, but not in the manner of having thousands of hours to grind by doing the same thing over & over & over & over.
It isnt fun.
Its lazy & unfun game design.