An Arduino is not an alternative, unless doing something very basic. It's not merely speed and RAM, but support for popular OSes and software (Android, and popular Linux distros), which even the fast expensive Intel board lacks. The RPi(s) is/are not (a) uC board(s); the Arduino(s) is/are; but the RPi(s) has/have GPIO anyway.
I generally agree with you.
The RPi is basically built at approximately minimum cost, and sold at near that price (give or take a bit). So it is excellent value for money.
But the specifics of some projects, make other systems (potentially including the direct variants of RPi, including various expansion and add on boards, etc), can sometimes make more sense.
This can mean spending huge amounts of money, e.g. $999 (Guess) just for the board, because it has the ultra high speed A to D converters, or huge amounts of Ram, or FPGA(s) built in, etc etc.
Or getting $3.99 small Arduinos, because your centipede (hypothetical) robot, has a micro board for each leg pair (LAZY tech development or highly modular construction) needs x50 boards per robot.
50 X $4 = $200 is much more affordable than 50 X $35 = $1750.
(The centipede is made up of 50 3D printed, plug together modular sections, which each have their own tiny micro board on them).
N.B. I just made up that example, but there are many real projects, which can be like that.
EDIT:
To be clear, I AGREE WITH WHAT YOU SAID.