The obvious hardware reason on iOS is that Apple can remove support for ARM32.
The obvious business reason is to encourage subscriptions to Apple Arcade.
Section "Why Drop 32 Bit?" (last quarter of the article).
This is business, nothing else. Just like all the increased "security" restrictions. Corralling everyone towards the App Store. I think they've got too much legacy to ever totally cut off "regular" applications, but apart from that they're doing everything they can to move everything in the direction of iOS (or iPadOS now).
Not sure how long the days of the last commercial Unix will last.
The Classic Environment (OS 9 -> OS X) was dropped after 6 years.
Rosetta (PPC -> i386) was dropped after 5 years.
i386 -> x86_64 is now being dropped after 12 years.
Running 32 bit apps in Mac OS X Mojave is 100% seamless to the user—if it wasn't for Apple's warning dialogues that 32bit was going to be deprecated, most users—even advanced users—would never even know the app was 32 bit.
And that's because 32 bit support is a key feature of the amd64 architecture—it's the main reason most computers today use amd64 instead of the alternate standard proposed by Intel.
Killing it in what way? They kept compatibility for years and eventually ended it, just like the transitions you mention.
This is business, nothing else
Both the security changes and dropping 32 bits are technically reasonable things to do. You might not like them and that's perhaps not entirely unreasonable either but the notion that they were done for business, nothing else makes little sense. What business do you have in mind? Apple makes money selling iPhones.
That's entirely the case. Apple has to weigh the cost of maintaining compatibility against the value it brings.
And it's hard to see how if the developer can't make a business case for updating their software that Apple can make a business case for maintaining compatibility.
That's the business reality of Apple's primarily consumer market: consumers are generally willing to let old software go and buy new stuff.
Microsoft is simply in a different market: they have many large enterprise customers who are willing to pay top dollar for extensive backwards compatibility.
I'm pretty sure Apple licensed that for some version of System 7 (maybe 7.5.something?) so that they could assume all post-boot 32-bit macs were 32-bit clean.