Guaranteed Victory for the Spiteful

When moving through an enemy's holdings, destroy absolutely everything. This ensures that, even if you lose, they will still have the hassle of going through and replacing all of that stuff. If we all band together, we can bankrupt evil through property destruction.

We may not be able to kill the big, bad, evil guy, but we can for sure ruin their weekend. 

There is a great deal of scholarly debate on the age of Deuteronomy. Religious tradition dates its writing to the 1400s BC, while more modern scholarship theorizes the work was first written sometime between the 700s and 500s BC.  The exact date or method of calculating the Book’s age is not relevant here - all that matters to this thread is that Deuteronomy is very old.

Why is the publication of a book from the Torah and Old Testament relevant to this thread? 

Independent of any religious computations - which are likewise not relevant to this thread and a topic not really appropriate for D&D Beyond - Deuteronomy contains our oldest written set of laws governing warfare. Among other prohibitions are commands that combatants do not destroy fruitful lands, only destroying or seizing property necessary to maintain warfare.

Later works expanded on these prohibitions, and for good reason. This kind of Sherman-esq destruction is utterly debilitating to local populations, greatly harming not only the lives of innocents during the war, but potentially having repercussions for generations to come.  Even beyond the cruelty, the wanton razing of land can hurt your own logistical network, turn potential assets into enemies, and breed contempt that can result in generational insurgencies.  Tactics like this hardly guarantee victory, particularly when one considers these mechanisms regularly result in the creation of new threats.

Thus, your proposal here is not only a war crime, it constitutes the oldest known war crime.  And it is simply bad policy that would cause significant problems in the short term and substantial problems long term.

I believe the alignment chart deems that "A dick move" but it is a valid tactic (see scorched earth warfare) but you do have to ask yourself this " Why am i fighting the evil king?"
Because that answer will perhaps point to why you shouldn't. If you are trying to free an oppressed people so they may have a future, you may want to leave certain things intact so that the population can support themselves after the King's downfall.

If however, this is just revenge and damn the costs, then an evil or deeply chaotic neutral character might do that. 

Real world sources and discussions of warcrimes are all well and good when it comes to real world stuff, but some of the spells i get as a druid would constitute warcrimes by the Geneva convention. Stinking cloud, cloud kill, contagion ETC would fall afoul of prohibitions on chemical and biological warfare.

Also you might spend so much time and resources destroying the buildings and holding that you weaken yourself. Spell slots are important if you want to be able to fight, and burning out a stone structure takes time.

Also if you salt the earth, then ever druid everywhere might have a few choice words to say about it.  

If you're going to waste hours destroying the furniture and linens in the evil overlord's fortress, be prepared for the evil overlord to call in heavy reinforcements that you'd have avoided had you opted not to spend so much time being petty.

The reality is that most wars are about either "We want your stuff" or "they want our stuff"

In the first case, laying waste to their lands is destroying the stuff you planned to loot.

In the second case, they're probably in your territory and so the stuff you'd be destroying is yours, not theirs.

None of this prevents a great deal of destruction in war, but it's almost all incidental to some other goal, rather than a primary objective.

I'm struggling to understand the allure of being this petty just in case the party gets TPK'd. If my party is expecting failure to this degree, I've done something terribly wrong as their DM.

The only way "wreck up the big bad's stuff so much it causes them logistical problems" sounds remotely fun is if the setup for the campaign is "you're Hogan's Heroes but in a D&D fantasy world." And honestly I don't know how someone would make the whole constantly breaking out of and back into a prison camp fun as a D&D campaign or what the endpoint would be, but it would be interesting to see someone try.