ElectronCharge already gave a good answer, but I would like to add a little extra information. There already exists 64bit only processors, as in they have no 32bit instructions or registers. The reason they didn't take off in the consumer market is because they simply cannot run 32bit programs or any existing consumer operating system. Intel sells these 64bit only machines to labs were they can actually make use of the full memory addressing, as in putting terabytes of data into RAM, and also only need 64bit operations.

The 256bit and 512bit registers and instructions are reserved for specialty hardware, IE graphics cards were you need to do floating point arithmetic.

One more aside, the reason we call 64bit processors AMD64 is because AMD subverted Intel by releasing an architecture which had both 32bit and 64bit instructions, while Intel was creating 64bit only processors.

Also I love this question, because it was the first question I asked in my kernel development class, but the answer I got was "all will be explained in due time"