The scenario assumes a few conveniences though.
1.) The berries last for 24 hours. The OP assumes you blow all your spell slots the night before, long rest, and then take the berries with you into the dungeon. Now, you're on the clock. A minimum of 8 hours is spent on the rest, and unless you camp out right in front of the dungeon's mouth, you have travel time as well. A DM can make things hard on the Goodberry producer by making sure the dungeon is more than a days travel and pitching a random encounter a day before they make it (having the simultaneous benefit of depleting berries and spells slots to make more).
2.) The stated benefit of this method is that the druid can keep his spell slots for the next day and still carry his full allotment of spells worth of berries. That works, for one day. The next day, if they group hasn't finished their objective, those berries aren't going to survive day 2. Furthermore, unless the druid didn't use a single spell slot in day 1 (defeating the purpose of casting it the night before), he can't create nearly as many berries for day 2 (greatly reducing the usefulness of the trick).
3.) Berry healing might top off a combat hp, but it doesn't eliminate a short rest. You still need those for encounter abilities. It just makes hit dice redundant by spreading out their benefit over every encounter instead of bursts twice or so.
This might play havoc on a AP like OotA, but home play can begin to adjust for said tactic in a lot of ways. More encounters (in the range of 7-10) and /or with harder foes (adjust to make them one category harder; no easy, more deadly) should stretch their limits. Add a few more monsters to an encounter, give the bad guy boss some extra goons, and up the number of traps and such. If your PCs insist on coming in with an extra day's worth of healing, then make it worth it to them.