Seriously? How is adding functionality to an existing power-button the same thing has having a separate, dedicated button that will only do one single thing (launch Bixby)? Its not the same thing, not at all.
You also have to remember you are comparing two vastly different platform. One being based on a open-source platform (Android) and one being based on a closed-source, locked-down platform (iOS). And this open-source platform (Android) gives you a bunch of other voice-assistants, like Google (Androids) very own Google Now / Google Assistant so it just stupid to have companies like Samsung locking down an entire button so it can only be used for Bixby when they insist on using a open platform like Android as their operating system. Especially when phone is sold in many markets where Bixby isn't even available.. Why would you sell a phone, with this dedicated button in markets where this button doesn't do jack as a result of Bixby not being available? It just doesn't make any sense.
This is Samsung in a nutshell. They've done this for years. They take the open-platform that is Android and puts tons of proprietary layers on top of it while they lock out basic Android features that compete with their own proprietary ones. Just like how Samsung got rid of the built-in native Chromecast/Google Cast support in Android 4 (or was it 5?) because they wanted people to use their own S-Beam/AnyCast/Screen Mirroring and whatever they have rebranded it as over the years.