That does look unhealthy on the bottom but the top surely looks good. Hard to tell how many years it has but I would not think it to be an emergency job. Still, one has to think about wind storms...
Not an emergency but It's only going to get worse . They have been thinking about taking down that sycamore and Maple in the back yard down for a long time, because the maple roots are causing some problems with their septic system.
Almost looks like it had a lightning strike along time ago, and the outside stayed alive while the inside died? Almost all sycamores of any size are hollow in the base, so it could last awhile.