I've talked about keeping my Sunday beer-and-pretzels, hack-and-slash game exactly that - beer-and-pretzels, hack-and-slash.
I've talked about campaign creep, too, and how I avoid that.
Part of this is, no quests.
There is a wizard in my campaign named Black Jans. He/she/it has a mysterious nature, a tower that appears and disappears from Stericksburg seemingly randomly (actually randomly - I roll dice for appearance), and strange servants.
Black Jans buys weird items, can enchant anything in one week for twice the cost (violating all sorts of rules about economics and how enchantment works), and provides a good excuse for the market of Stericksburg accepting cursed items and blacklisted books for cash.
We also have a big church that can perform Resurrection and cast Remove Curse, a bevy of nobles, cultists the players have clashed with (and even, once, arranged a deal with), and guilds of every stripe needed to justify in-town rolls.
Occasionally, though, the players wonder - does Black Jans have anything he/she/it needs done in the dungeon? Does the Thieves' Guild have any quests for us? Can we ask the church if they have sometime they want done in the dungeon in exchange for money?
My answer to all of these is, basically, NO.
If the answer becomes yes, then the game shifts from a player-driven sandbox to a GM-driven sandbox.
It's one thing for people to offer special rewards - way, way, way back early in the game one noble offered to pay more than the cash value of good from one of the draugr for its return. Prince Vlashkalabash the III of Cashamash has a reward out for finding and giving unto him Gram, the dragon-slaying sword. Sometimes people offer a bounty for specific goods or items.
Those are okay because the PCs can act on them or not. They're just bait from me, the GM, to the players, to incentivize certain activities or to up the potential awards from those actions. Or to hint that certain things exist. Or to put up a choice - do they keep Gram when they find it again, or sell it for a fortune?
But once the PCs can go to town and start asking around for quests, then it's really up to me, the GM, what they should do in the dungeon. Its an easy out from making your own plans and decisions to saying, hey, GM, tell us what to do. Even if that's not what is intended, it is inevitable it will happen.
After all, quests will come with additional awards. Or come with punishments for not doing them. This limits your actions and gives an incentive to seek them out. Why make your own plans and own decisions when someone in town can tell you what to do, possibly tell you things you didn't know, and then give you extra money for getting it done?
Failure - which is fairly common - means complications. Those complications must be town-centered since the quest origin is town-centered. That means town suddenly acquires more depth because the PCs are having trouble in the dungeon. Social relationships in town acquire more depth with failure. Say Black Jans sends you on a quest and you fail. Either services in town end, or you have to avoid town, or you have to make up for it with another quest. What if - and this has happened in games I've run before - the PCs fail or just find out it's harder than they thought and demand more support or more loot? Social relationships in town deepen or end. Town becomes more important.
You can still seek sponsors and find people and propose special awards, if you're confident of your skill roll results. But you are generally better off doing your own thing. No one helps you for free, and that is not only realistic but helps drive player-centered play.
And yes, you can find people in the dungeon and do things for them. You can ask them what to do. Because any complications that result from this are directly impacting the fun part of play (the dungeon). Bad results from success or failure impact the PCs in the play area. The players dealing with those consequences are all within the play area. Doing favors for Faction A and killing off Faction B has consequences when Faction A isn't your friend anymore.
That's why "No Quests!" is an important part of keeping my game beer-and-pretzels, hack-and-slash, and a player-centered megadungeon sandbox.