I find it hilarious that old keyboards used to have most of these buttons. Gradually people migrated to "60% TKL" keyboards to save space, and now apparrently buy an entire stream deck to replace their missing buttons?
I admit it looks cooler though
Hmmm, I only recall one previous "proper" keyboard having OLEDs under the keycaps...
Slightly facetious, I know, but I think one key element you missed is the adaptability.
Yes keyboards used to come with calculator and mail and internet buttons, but often times you were stuck with those options only, or worse, the particular apps hardwired in the drivers.
Not only can this solution adapt to what you are doing / context aware, it can give feedback. That mail button can pull and render an inbox count. Online contacts profile pics can display in a select few buttons when they are available.
Price/gamer tax on this type of solution and how useful to the average user, that's up for debate.
Although my normal keyboard does still have function keys and a numpad, I like being able to map annoying shortcuts to individual buttons. For example, Photoshop's "Merge Visible Layers" (default Ctrl+Shift+E) is hell on my wrist, so I map it to a button on the MIDI controller.
It's also nice having extra buttons to access F13-F24 for whatever behavior I want since virtually nothing uses them by default.