20-Year Prospect
It's probably tempting to think that in twenty years we won't have ice left. While we'll have less ice in the wild, we'll definitely still have it indoors, sort of like those endangered species breeding programs in the zoo. As long as winter sports remain popular, there will be a need for smooth ice, unless the sport is "let's see how many figure skaters we can horrifically injure."
Popularity in these sports is declining in the U.S., but, other countries still love them (source). Maybe it's because they haven't heard of basketball yet, we're not going to speculate. So if your dream is to drive a giant machine that pounds ice, you might have to move to Canada.
The actual Zamboni is already changing. While it used to use a complex system of hot water and brushes, it's started to incorporate laser technology. Additionally, Zambonis used to have internal combustion engines, which are generally a bad idea to drive indoors. Toronto has switched over to electrical (source), something that will likely become a trend.
It doesn't look like the machine itself is going anywhere in the next two decades, eventually the Zamboni will exist no more. It will live only on nature preserves, trundling about on frozen lakes, fruitlessly chasing skaters. Switching over to automation isn't much of a stretch either. While that's true for pretty much every job, in this case there's already the Roomba showing where the technology can take us. Right now the size and complexity of the machinery make this impossible (source), but there's no reason to think that might not change.
For now, the job is secure. But beware the possibly imminent Zambroomba.