Why would someone go get an Apple when this is an Android phone? Design is merely one aspect of the overall device. Changing to another OS is something else entirely.
There are other Android devices that run the same design for multiple years, but still run the same fundamental OS, Samsung being the most notable. Their current design started back in the S7, which started out as a somewhat large bezel phone and then shrunk the bezels with each following model, same as the Pixel line has done, and they're in their third year of this design. HTC did it as well with the One M7 through the M9 line, and if anything they changed their phone less through each progressive design than Google has done with the Pixels. When these OEMs have iterative designs, it seems to run in a 3 year cycle, which is where the Pixel is now.
Some have brought up that it's a branding thing, and that's probably a big reason why they are doing what they are. They didn't have to worry about branding with the Nexus line, because it was directed at a small group of developers and tech geeks. But when they switched to the Pixel line, they were targeting the general public, and having a recognizable design is key to growing a brand with people who are less involved in the intricacies of the industry.
There are lots of other companies that do that. Car companies often times have certain body parts that are instantly recognizable as being from a specific manufacturer, and many times those design cues carry on for decades. Hell, listen to an AC/DC or Queen song and you'll note the signature guitar riffs that are inextricably linked to that band. The thing is on something like a smartphone, there isn't a whole lot of room to differentiate from other OEMs' offerings, let alone their own. And if they take it too far, they risk alienating those who liked what you have previously done (the Band-Aid design of the Galaxy S5 or the rear design of the LG G5 for example).
Design is a big part of a smartphone, but it isn't the only part. Few people are going to buy a phone with a killer design if the rest is lacking, but more are likely to buy a phone with a bland design if the rest is worthwhile.
If you don't want it to change, that's fine. I like change. Surprised so many people are upset by this. Go get an apple if you want the exact same device with a processor bump.