This twice-raised thread merits however, a question. Is turning a select stone pillar to some common material like balsa enough to collapse a significant part of a build over the duration of the spell? Or would the frailty of the load-bearing element produce effect after a long time?
Not really. Causing serious structural damage to buildings usually requires high-level spells.
Most buildings generally won't collapse if they lose a single load-bearing structure like a pillar, and turning one pillar or a part of a wall to a softer substance for an hour (or even a day) wouldn't really damage it much due to material stress - for things like that to be seriously damaged, you need a lot of stress over a lot of time... Most buildings that collapse do so due to serious material fatigue over the course of
decades.
In order to sabotage a stone building, you'd have to repeatedly spend several hours transmuting load-bearing sections of the building materials into a softer substance and damage them with some other method so that they remain damaged/destroyed when they revert back. It would probably take an entire week or more of work to get a sizeable enough area of the building weakened enough that it would have even a chance of
eventually failing structurally due to material stress, depending on the construction and materials involved and what method you're using to damage them. Which may or may not be enough to cause any undamaged areas to fail as well.
So there's not much chance of succeeding at dropping the dungeon's ceiling on some goblins while they're sleeping, but if you can spare several weeks of uninterrupted time hanging out in the sewer tunnels under a building, you could theoretically weaken the material above you enough that it would
eventually collapse, potentially bringing the building above it down as well. Which is more of a plot device sort of thing rather than a strategic plan for adventurers, but still...
Minor Transmutation
is, however, a perfectly feasible way to knock a simple hole through a stone wall if you have the time to do it and sufficient oil/fire, damage cantrips or an axe... Or to get into a locked iron strongbox if you can't pick the lock.