Kinda. It's kinda of a cop-out. Which is a nice pun since we're talking about talking to cops of the medieval world.

In the end, it's about preps. If I prepare an adventure, I will not prep it with and without guards just in case they go and convince a department to come down the sewers to stop the cult with them. I'll only prepare one of them. If they go for the other, then I have to refuse for some reason or to redoe my preps, sometimes in the middle of the session. That, or you make the contribution insignificant. If you go down the sewers with the guards, then I would make them fight some cultists in the background while they fight the encounters I prepared for this number of combattants of this level.

Now don't get me wrong, I'm not saying : I refuse anything that wasn't prepped in advance. Because of course I don't do that, that would be railroading. But at the same time, we all mostly agree in the TTRPG circles that it's a douche move to not do what the DM prepared for that session. The classic example is having Dracula's castle right there, but the players decide to go in the forest instead for no ingame reason. Should the DM improvise an entire other adventure for them right there and then ? Should he tell them there's nothing in there ? Should he let them wander off for no reasons ?

I think the sweet spot is between these 2. Between the players wanting to go randomly in the forest and the DM refusing to budge from his preps. You should devide where you wanna go and why as the players AND respect the preps the DM actually did. If you go off the preps, don't expect anything of quality already good to go. And as a DM, you should allow players to do stuff outside your preps as long as it fits your improvising skill and enjoyment and (ideally) doesn't make you waste hours of work. Because that stings so bad it takes away my will to even begin those preps.

And for me, calling the guards to your help when the quest never mentionned them or even needed them in the first place is big. Really big. Should you make guards with shitty blockstats to let the players shine and be cool ? Should they be overpowered and deal with the situation without help ? Should they be as good as the players and put in question why they are even needed ?

Lots of questions that I don't really have fun answering live during a game.

I'm curious for your example thought : Guards finds who gave them their quest tells them to hire a better adventuring party because again why not go to guard in first place if not?

Well, why not go to the guards in the first place then ? The answer out of game is obvious : the fun is having a quest be done by the players. Ingame, it would be a reason that the person cannot go to the guards. Which sure, you can plan ahead in case your players have the very bad habit to go and get help everytime they have a challenge to do. But it's more work. More work for the DM already doing so much for everyone's fun, including his own.

So you are correct. It's a cop-out. But if I were to always plan everything all the time just in case my players went to the guards, I'd probably eventually just say fuck it, and tell my players guards will not help you solve quests because I do not want to keep doing this.

And I think calling upon the guards is like going into a forest randomly. And that how YOU deal with this at YOUR table with YOUR players are 3 things that will make the answer to : what do you do when this happen ? very differents. My players, at my table, will be dealt a certain way with these things that will not work elsewhere. Because it's tailored made for them.

In the end, remember. If you're not having fun doing something, don't do it. Even if that thing is always having to find a reason for why guards won't help you.