"I have not seen these things be successful, therefore they can't be successful".
This kind of thinking scares me, not so much because it's so myopic, but because it becomes a self-fulfilling prophecy: "I have never seen success" becomes "Success is not possible" which becomes true because we stop trying.
Mere mortals throw up their hands in futility, blaming the user, the state of the art, or the alignment of the planets, without ever understanding the fundamental principal that great systems accommodate great needs and the greatest need of all is constant change.
"the company recategorizes its product line based on utility and lifestyle rather than brand and designer"
This is incredibly common and can easily be accommodated with proper database design, decoupling products from their associated attributes.
"the parent company divests itself of you and withdraws the connection to its parts and inventory database"
So now you have to treat them like any other vendor? You do have some of those, don't you?
"a manufacturer starts using more than one UPC per product because they changed the packaging"
A simple database design consideration. "One to one" is just a special case of "one to many".
"the law now requires that national holidays must be counted against vacation time if the employee takes a vacation day immediately before/after that holiday"
Rule driven logic. What's the big deal? (Your logic was rule-driven from the outset, not hard-coded, right?)
"a loan company files to become a bank and store deposits that earn interest"
Additional logic to existing data. We do this all the time.
"CSV files exchanged with a partner were always denormalized, but then one company starts using normalized records with mixed column mappings"
Any decent batch file processor should be able to handle anything thrown at it. Companies that do a lot of this build uncertainty into the design of their systems.
"another partner decides to ditch file transfer as a way to submit data, and creates a web service that you're supposed to invoke commands on"
Sounds like you'll need an extra piece of software to create a feed. Not exactly like no one's ever done this before.
"the shipping carrier reveals that their tracking numbers are all recycled at the end of each year"
Not a problem if they were never used as the primary key to a table.
"new rules require every credit card number to be encrypted, but you still have to be able to quickly search for matches in a database of millions"
Ahh, but the last 4 digits do not have to be encrypted. Start your search with them and resolve synonyms. Pretty standard stuff.
"a partner switches to mixed-case alphanumeric PO numbers"
As an identifier, a PO number is always a string even if it looks like it's numeric. (When was the last time you added 2 PO numbers together?)
OP's roadblocks are experienced developers' speed bumps. Just because he hasn't done it doesn't really mean that "Nobody knows what they're doing".