This is somewhat reviving of an old thread
As of today there are no supported architectures with >64 bit pointers. There aren't any on the horizon and that doesn't seem likely to change in the foreseeable future.
At the time of the above thread there wasn't a way to have write architecture specific trait implementations. There now is. So we could implement From for u64 today, and if a 128 bit architecture is introduced in the future, the implementation can be made architecture specific.
Obviously this would have the consequence of causing code making the assumption that this conversion can be preformed lossless to not build on 128bit architectures. However I think this is preferable to what is happening now where this trait is not available. I don't think because the trait is unavailable people are actually coding to the assumption that a u64 might not hold a usize, instead people write try_into().unwarp() which would result in a failure at runtime or worse as u64 which would hide the bug all together.