Marlin barely fits on the Arduino anymore, and that's why recent releases also target ARM and ESP32.
In any case, my point is that the Atmel Arduino architecture is old and limited, and imposes a number of design constraints. There's hardware that's both better and cheaper out there now.
You're talking about AVR8, which is the ATMega328p and its children and siblings. There exists, though less popular, a complete rebuild of the AVR architecture in 32-bits called AVR32. The instruction set is different, the peripheral IO is different, but the design is similar. The AVR32 was powerful enough for quite some time to run a full fat Linux, though this was dropped for a handful of reasons.
In any case, my point is that the Atmel Arduino architecture is old and limited, and imposes a number of design constraints. There's hardware that's both better and cheaper out there now.