I mostly had this idea because of Fairy Mushrooms.

I'm sure everybody, at one point or another, got tired of having to use the same healing items over and over for the same effect.

What if there was a 'Mass Use' button that let you choose how much of the stack to apply?

I mean, fairy mushrooms are... basically only used as CoD fodder by most people (since they're the most useless healing item in the game).
Once you've been playing for a while, people who do a lot of missions tend to end up with plenty of higher-level potions and elixirs (strong restore potion/elixir II and III heal 1600 and 2400 HP respectively.) which can heal most pets of any damage in 1-2 (and/or they have a decent light healer that can fix any damage if they take their team against a non-boss enemy)

There also are some specific mass-consume items that use the whole stack at once (most of not all of those are fest items I think?), and you can just split the stack if you don't want to use them all



Aside from all that, many people have spent a lot of time calling for a mass-use option for things like mission satchels, so it isn't like it's a bad or unwanted idea.
Some items could definitely use it, and the 'select how many' bit would maybe? go faster than stack splitting and then using (though Krin did just add the split-stack button to the inventory to save everyone the trouble of the previous effort it required for some people like those who used mobile or certain kinds of computer)
If I could mass use mission satchels (or any kind of satchels) in any amount it'd be nice.

The problem with mass using satchels is that they are randomized contents.

Imagine someone trying to open 100+ mission satchels at once.

I could see it working if there were a limit to the number of satchels that could be opened, like say only 10 at a time, but being able to open an entire stack of them?

Most of the mass use items are ones that have either the exact same contents, so it is a simple matter of multiplying the number of satchels opened by the items gained, or they are very limited on randomizing, such as the brewing agents. You have three outcomes, so it is relatively easy to mass open those because you just have to figure out how much of any particular agent you are going to get.

However, when you try to mass open items that can give say 5 outcomes, and can give just about any item on site? that would cause a lot of lag.