Google already took this leap back in 2015, dumping 32-bit versions of Chrome for Linux.
Ubuntu is just the latest distro to plan for this
Ubuntu’s Dimitri John Ledkov put forth a proposal to wind down 32-bit support on the Ubuntu mailing list recently. Hardware that can’t run 64-bit software is becoming much less common, while creating 32-bit images, testing them, and supporting them takes time and effort. (On Linux, the “i386” architecture is the standard 32-bit for Intel-compatible CPUs, while “amd64” is the 64-bit architecture originally made by AMD that Intel CPUs are compatible with.)
Ledkov points out that Ubuntu wants to limit the number of new 32-bit installations, with Ubuntu 16.10. This next release will not offer a 32-bit Ubuntu Desktop or Ubuntu Server image. The software could still be installed for legacy compatibility purposes via more traditional installers. By Ubuntu 18.10 in October 2018, Ubuntu would completely end support for 32-bit software and encourage running it in a virtual machine or container instead.
Fedora has made similar choices. Fedora Server no longer offers a 32-bit image as of Fedora 24. A proposal to dump all 32-bit images in Fedora 24 failed, and Fedora 24 still offers 32-bit versions of Fedora Workstation. However, Fedora no longer considers them important. As the developers put it during the meeting, no one wants to support the 32-bit images. I’d expect to see Fedora stop releasing 32-bit images within the next few releases, too.
OpenSUSE Leap hasn’t even offered 32-bit images since its inception. OpenSUSE Chairman Richard Brown explained on Reddit that it wasn’t worth supporting 32-bit for another three years after the release because downloads of 32-bit releases have steadily declined. “I know some people passionately enjoy their old 32-bit hardware, but I think now’s the time to consider letting it go,” he wrote.
32-bit Linux distributions will still exist
Thankfully, 32-bit Linux distributions will continue to exist for a long time. The recently released Ubuntu 16.04 LTS will be supported for five years until 2021. Even after many of the big Linux distributions stop releasing new versions for 32-bit hardware, there will be Linux distributions out there specializing in support for this older hardware. But, by then, more 32-bit hardware will be out of commission.