Dropping support for i386 hardware and installation: Fine!
Dropping support for i386 applications: Likely going to be a mess!
And no, requiring people to run containers and VMs with old OS’s does not count as “supported”. That is exactly why we have DOSbox, WINE etc; to be able to run decades old software on our fresh OS’s.
Hopefully a smoother solution is found than LXD. It might be OK to encapsulate i386 to using an older subset of libraries. However this should be supported way beyond the support period of 18.04.
Maybe using Debian i386 libraries is a solution?
But prodding developers to getting their apps to 64-bit now(!) is a good thing 