Microsoft is working in Windows 10 Mobile 64 Bit for Surface Phones 2017.
"In March 2005, he completed one of his "most gratifying pieces of work" at Microsoft when, partnering with AMD, he helped develop the AMD64 architecture (64-bit extensions to the 32-bit x86 architecture) and led the effort to ship the first two x64 64-bit Windows systems (workstation and server). At the time, some questioned why Microsoft developed a 64-bit system; today most computers are 64-bit systems and even our phones will soon have a 64-bit operating system." Source: Microsoft
Microsoft is truly readying the Surface Phone lineup for 2017 and is working on a 64Bit version of Windows 10 Mobile and it simply makes sense because it allows for better performance and more powerful hardware. The real gain is the boost in performance possible thanks to more RAM, (more than 4GB RAM) which could finally make Windows 10 Mobile devices more appealing in the enterprise.
Rumors are saying Microsoft will launch 3 Surface Phone models in 2017 and one of them will be for business customers.