The switches I'm talking about are small enough that I could literally fit one them inside the smallest button if I bored out the inside. The ones on the controller already that are used for the bumpers are about as thick as the rubber membrane itself.
Show Image
(https://i.imgur.com/wZrYEV2.jpg)
How is it not practical?
Those small smd microswitches are not very reliable, because of how little copper they have internally.
Their tactility/feel drops sharply with use.
Typically, if button switches of that type are used in high repetition areas, an over travel prevention mechanism will be built into the slider. (similar to logitech mice H stem button flaps)
It's the primary reason they still use silicone caps, it's much more durable because the thumb grip is very strong.
Also with low travel such as this, the difference of 0.1mm makes a big difference in feel.
Unless you have a precise milling machine, it's hard to get the buttons to feel even.