Not an inspector or electrician ... but I do have that bit of libertarian in me as well. When a home owner upgrades something, he/she should never be forced to upgrade something else. If the a change in a circuit would not require a change in a breaker, the breaker should not be otherwise changed. But if the breaker has to be changed, and AFCI is available, it should be changed to AFCI.
As for multiwire, I would simply classify that as "AFCI is not available", except for the CH panels where I have seen 2-pole AFCIs in their catalog (maybe BR panels have them, too). Square-D and others need to get on board and quit making excuses, or else completely fund a nationwide "replace every residential multiwire" campaign.
There are numerous scenarios where people would simply not do upgrades at all if forced to "over upgrade" or disallowed to "incrementally upgrade". We should want homeowners to upgrade as much as they can afford. One scenario I had mentioned a long time ago was the case where someone with an old bathroom mirror/light/outlet combo that had ungrounded unprotected outlet on a 15 amp #14 circuit could not make this "incremental upgrade" by adding a GFCI outlet in the wall next to this mirror thing, because they would be forced to upgrade the wiring to allow 20 amp to meet the 20 amp requirement. Basically the code, if fully enforced for upgrades, would be forcing someone to stay more unsafe than they could be.
A landlord renting residential is another story. I'm all for requiring them to meet the current code every time a tenancy change takes place, including a re-inspect if that hasn't happened in the past X years.
As for a home being sold, it is a frequent case of homes not meeting today's code. A list of every upgrade needed to meet current code should be provided to the buyer before closing, and buyer allowed to withdraw on the basis of electrical code issue if the seller won't correct or deduct estimated value from price. I don't mind buying a house out of compliance as long as I know exactly what I'm getting into and what it will cost me to correct it, and a discount to cover that. I'd actually like buying a home where I have to do a total rewire since then I'd know even the hidden parts are now done right by the people I'd hire to do it.
