is there any reason why units cannot be disbanded in lieu of building, say for example, to avoid going into a treasury deficit which would require increased unrest to balance the books and, consequently perhaps, causing a player to lose the game as a result
I assume that you want to do this probably in your last Action Round to avoid the payment problem soon to come up in the Income and maintenance Phase? [e.g. you are going to earn 3 gold for your population level of 3 and 4 gold for 4 control markers = 7 gold, but you have 8 units on the board, so will end with a deficit of 1 and take 1 Unrest.
 It is not legal to take a Build Action and simply destroy a unit [whether Army, Fleet or Fort.] The only time that you can destroy a unit is when you want to build a unit that you no longer have any of in your supply.
 Also, the taking the Build action and destroying 1 unit, even if legal, would be a waste of time. Destroying a unit causes you to take 1 Unrest Token.
Thus, you still end up with 1 Unrest token!