In practice, how much of my code is going to be run on an 8-bit CPU? Zero. I'd rather have the clarity and consistent behavior of explicitly typed ints than pre-optimize for something that will almost certainly never happen.
There were so many bad programmers who assumed 32 bit machines would last forever, that now we're stuck with compilers that don't default to 64 bit ints even building for 64 bit targets.
This will bite you if your code depends on int being 32 bit (for example if you depend on overflows or use asm). If your code explicitly uses an int32, this will always be true.
Silently increasing the size of an int is more likely to break things than it'll be helpful. If you really want an int to be 32 bit on a 32 bit machine and 64 bit on a 64 bit machine, make it explicit (e.g. intptr_t for pointers)