In most other cars (say most non-performance oriented cars), a platinum plug need be changed around 60 to 90k (Iridium need be changed around 100k). Audi's recommendation of change at 40k sounds reasonable as most German cars are some sort of performance cars. And, knowing most German cars have minor oil consumption thus there is certain concern of the possibility of plug foil even with relative hot Platinum tip. Plug replacement is one of the easiest and one of the relative straightforward maintenance. Platinum plugs are relatively cheaper than iridium plugs. The tricky one is DSG fluid change at 40k. It's not so easy for DIY and quite costly in dealer service.

Agree with most of what you're saying, but classifying the a4 as a performance car is a bit stretch imho, lets look at the a4 2.0 vs the h-accord 2.0 only motor, why the plugs last longer in the Honda and not the Audi which i love? would you call the Honda 2.0 a performance motor too? btw we're just reasoning to learn.

Honda accord uses the exact same Iridium plug for both 1.5L turbo and 2.0L turbo engines in all trims including LX, EX-L and Touring models. You can see their priority is in Economy rather than performance. And Honda is in kind of "disarray" in recent years.
Talking about plug replacement interval, driving habit affects the plug longevity greatly. For mellow driving owners, you surely can stretch the plug replacement interval to 60K without any issue. Japanese manufacturers have sever and normal driving condition maintenance schedules. European auto makers have only one schedule which all fall into sort of sever driving condition category.

100% in agreement.Economy as in cost cutting, yes, engine to engine they are both apples to apples and if you ask me Honda has the edge by making the same power on regular gas, their sales numbers could suggest different,
Also, i found out from honda, plugs and transmission fluid should be changes at 47k in sever conditions, which is in agreement with your reasoning.

FWIW, I’ve been through this lifetime plugs, lifetime fluid thing. With my Honda and my Subaru. I use an indie mechanic because he is good and honest, unlike the dealer. He laughed every time I told him about that. He looked it up in his service that he uses and his service (here I have to apologize because I can’t remember the name of it but it was a well known name) gave mileage figures to change all of of the fluids in both cars. So, next i went to the dealers. They laughed and said there is no such thing as lifetime differential fluid, or tranny fluid, or sparkplugs or anything. They all had a form printed up that contradicted the factory manual, but had their recommendation for service intervals. For instance: Subaru says the tranny and the differential fluid are lifetime. The dealer and my mechanic said if you even try to do that be ready to replace the diff’s and the tranny. My mechanic said he just replaced the front diff in two Subaru’s due to not ever changing their fluid. Now, i take the dealer with a grain of salt, they make a lot of money off service. So I called Subaru USA and Honda USA, who said the lifetime fluids, plugs, etc., are only for people that drive under normal circumstances, which by their definition no one does. Everyone else should follow what the dealer says.

"Lifetime" parts often now mean that when they fail, their lifetime was up. But proper maintenance can make that "lifetime" last longer. Most car owners these days don't keep cars for extended periods, so the manufacturers only worry about keeping them running with low maintenance costs until most original owners have moved on. That leaves either the outlier extended length owners or, more likely, used car buyers to recognize the fallacy of "lifetime" and take appropriate preventive maintenance.

i completely agree with your statement.