No reasonable economists claims that GDP will grow steadily but there are good reasons to think it will grow over the long term.

GDP is the total output of a population. There are basically two ways it can change; you can change the number of people or you can change the output of those people.

In the long run, populations go up. People have been talking about variations of the Malthusian trap for centuries but the world population has increased by nearly 100x since then and growth seems to be accelerating rather than slowing down.

Individual productivity also seems to be on a firm upward trajectory. This is primarily driven by technological advances. A modern combine can harvest 200,000 pounds of corn an hour, that's about a 2000x improvement over working by hand. I have a 3-d printer on my desk that can build arbitrary shaped objects in a few hours rather than taking months to build prototypes. And we're all aware of the blistering rate at which electronics improve. There's no obvious reason to think this technological advance will slow down any time soon.

An other factor in individual output is hours worked. That's gone up a little but nowhere near as much. A few people are able to sustain 80 hour weeks but most people work 27-50 hours per week There's a lot of opposition to working 60+ hours per week and many people argue that we shouldn't even be working 40 hours per week but very few adults reasonably expect to work less than 20 hours per week. So we're unlikely to get much productivity reduction out of change in hours worked.

Is this desirable? Absolutely. Increased output means that we can get more of the things we want and need. For many people on earth that still means basic things like food, clothing and shelter. Many others require things like improved transportation, healthcare and childcare. Even the rich have desires that they can't meet yet.

The bigger questions are if we can support sustainable growth and how we distribute that increased output. If we could get infinite economic growth with no pollution, resource depletion or other externalizations it's hard to imagine why we wouldn't but that's not the real world. We absolutely have to deal with these issues. Either we deal with them preemptively or we wait until we're forced to deal with the effects after the fact. It looks like we're on a course to do a little of both. Resource distribution is also a very difficult problem. Nobody has proposed a system that everybody agrees with. We'll probably keep trying new systems indefinitely in the vain hope that we can find such a system.