Not really non-replaceable. The USAF now has KC-46's. Those are built on the Boeing 767 airframe.
Yes, and they deployed some of them for this operation. Then, just the other day, they scrambled another full dozen of KC-135s to the Middle East.
Yes, the KC-46, a 25-year continuing boondoggle that has never worked right and probably never will.
Yes, the KC-46, the "modern" replacement, based on the 767, an airframe that is 45 years old.
Tell me all about those lovely KC-46es. The ones that can actually still be produced, because Boeing is keeping a single line open with 1980s tooling so they can make... KC-46es and charge whatever the hell they feel like for them.
These lovely KC-46s, where for every one we've committed to buy you could have a random contractor convert an A350 or 777 to be hilariously superior for about half the price.
Anecdata: a currently active KC-135 operator in-theater told me that pretty much ALL the tanking is being done by -135s. Punch up a flight tracker, you'll see. Also, that crash? May just have been an in-flight collision because they are having all of these suckers do racetracks with exhausted crews and guided by PFC Maga SurvivedThePurge.
ETA: If you think SLS and EUS are the only projects Boeing has been massively shafting tax-payers on, look into the KC-46 project.