iPhoto did not "become" Photos. They're two different programs. There was an iPhoto for iOS, separate from the Photos app that comes baked into the OS, and the new Mac app is intended to be the companion app for the iOS app - to access your photos on either iOS or Mac, the go-to app is now Photos.
"Mac" did not become "iMac," or vice versa. iMac is one product line within the Mac universe.
iDon't know how or why people came to think the 'i' means "iNternet" - the iNternet had nothing to do with the first iProduct, iPod. We have iTunes because it was the companiOn app for iPod. iThink 'i' was iNtended to iNfer "personal," me, myself, and i. All through the day, iMe mine, iMe mine, iMe mine... (geez, Beatles lyrics... i'M showing my age.)
iThink, iN the late 1990s, when Macs had become better known as machiNes for mediA professiOnals, Apple needed to reiNtroduce Mac as a consumer product, and so associAted the new entry-level, put one on your child's bedroom desk Mac with Apple's other consumer-focused product. And of course, you sync your iPod to your iMac with iTunes... iPhone is an extension of Apple's personal electronics product lines. It even incorporates an iPod. iPad is, of course, an extension of the iPhone product line. iCloud plays on the consumer focus - there was a time, not so long ago, when "the cloud" was associated solely with corporate computing.
As to Apple Watch? Yeah, it beaks from that tradition. But so did Apple TV (of course, ITV is a British TV company, so iTV would have been an uphill battle in the courts). It's certainly an extension of iPhone, and can play music like an iPod, but at this point, consumers can't be sure that 'i' is for Apple, and a $17,000 watch isn't exactly a playful, entry-level consumer product. "Apple Watch" makes it clear that this is Apple's watch - trading on and reinforcing the company's overall image, rather than just one part of it.
And there's that problem with noun/verb confusion... I watch, I pay