Impossible. x64 without x86 compatibility would have gone nowhere in 2003, as at that point there were quite a lot of more modern 64 Bits ISAs to choose from (At least those few that were still alive after Itanium vaporware attack), and x64 for sure would have been inferior to any of them due to being built on top of x86 late 70's design idiosyncracies. Besides, AMD did some cleanup cause Long Mode deprecates the Segmentation Unit (The 286 MMU had a Segmentation Unit while the 386 added a Paging Unit, and BOTH could be used at the same time, which is going bonkers. Early 32 Bits virtualization Software before Intel VT-x and AMD-V actually used both for a technique known as Ring Depriviledging), and removed the useless TSS (Hardware Task Switching, which for some reason was slower than doing the same via Software, making it redundant). I think Long Mode also favours SSE over x87 FPU instructions, but if I recall correctly, they were still fully supported.The 'opportunity' for a clean break was their with x64 - but with two competitors in the mix (AMD/Intel) neither could afford to write off 32b instructions and APIs without losing out terribly to the the other company.
People are also forgetting that x86-64 was AMD desperate attempt to create a 64 Bits extension to x86 that would allow it to keep being relevant. If Intel succedeed in moving the PC ecosystem to IA64 as it wanted to, AMD x86 license would severely lose value, and you can bet that Intel wouldn't have licensed IA64 so that it could create the total monopoly it couldn't with x86. And AMD won precisely due to backwards compatibility. If you can get the important parts of 64 Bits (Mainly larger address space) in a Processor that also runs standard 32 Bits x86 faster than the previous generation, why bother with something else?
Intel actually had to catch up to AMD by keeping as a secret Plan B of sorts Yamhill, which was x86-64/EM64T support in Prescott. And eventually, Intel began to enable it in an increasing amount of models due to Opteron/A64 success.