Prior to the industrial revolution, it wasn't common that economies would grow in the medium term. Generally the size of the economy would be based off the resource base you had (farm land, functioning mines). Adding more people wouldn't produce that many more resources, so as population grew wealth per person plummeted.
Any wealth one person had was an equal amount of wealth another did not. The economy was a zero-sum game for the most part.
Governments would engage in wars, or build plague-filled cities, to kill the excess population; and if you didn't, plague and famine would come anyway and kill the excess.
Some areas where a bit wealthier, others a bit poorer, based on recent war results and local resources and the like.
The industrial revolution, starting in the Netherlands. For almost a century the area got slowly wealthier faster than it gained in population. Then England (and later the rest of Britain) followed suit.
Canals, steam engines, coal, wind and rails caused constant regular economic growth. Manufacturing and excess wealth caused cities to be a less crappy option, and manufacturing (textiles, ships, etc) started growing economically compared to agriculture.
The power of exponential growth means that a nation that grows 1% per year faster than its rivals, after 100 years is 2.7 times more powerful. After 200 years it is 7 times more powerful. After 300 years it is 20 times more powerful.
And the industrial revolution, while it started off at a sustained pace of 1% or so per year, accelerated instead. In the 1700s it was 1% per capita growth, then by the 1800s it hit 2% per capita growth. And a population boom also occurred.
In 1600, the UK was a backwater island of a backwater continent. By 1900, a mere 300 years later, the UK was a world-spanning superpower. It did this on the heels of 1% to 2% per-capita growth per year.
The modern study of economics comes out of Europe, and the countries of Europe either mimiced the industrial revolution or became conquered provinces of nations that did. It did not go well for nations that didn't manage this year on year growth.
So economics understands how economies expand, and describes disasters when they don't, because the science of economics was built in a nation that did exactly that (grew), and observed exactly that (lack of growth destroyed countries).
The idea that "not everyone can be a millionaire" is a bit vague.
Do you mean "not everyone can be richer than everyone else", where we use "millionaire" to mean relatively rich? That is true under any universal exchange rate.
If you mean "not everyone can be as rich in resources as a person in the west is that is currently worth a million US dollars", then that may or may not be true.
Suppose the sustained growth in the UK between 1700 and 2000 was about 2.5% per capita per year, and 1.025^300 is about 1600x. If we start with the average person being "worth" the equivalent of 100$ and earning about 1$ in current dollars per day (about 400$ per year), at the end of 300 years of 2.5% growth the average person is worth 160,000$ and earns 64000$ per year. (Note that the growth rate I wrote is a bit high; it was closer to 2%)
Do that for another 100 years and it becomes a worth of 1.9 million and earning 750,000$ per year.
You may wonder how that could be plausible.
A situation where batteries and solar power are 1000x cheaper than they are today is far from certain, but far from impossible. In that scenario, green house gas emissions are no longer a serious problem; instead, we run into net heat flux limits, where the power we use generates heat directly, and radiating that into space is the limit we have.
Before we hit that limit, we'd end up with a society where a good chunk of the planet is wrapped in solar panels (say, 10%) at high efficiency (say 30%), converting 3% of incoming sunlight into useful power.
Using the Kardashev scale, 2018 is about 0.73. A civilization that harnesses 3% of incoming light from the Sun is 0.85 on the scale, 16x "richer" in useful power than our current society.
Even ignoring the idea of using power more efficiently to produce wealth than we do right now, there is room to hit 2.5% annual growth over 100 years with a 16x multiplier.
The worlds current average GDP at PPP is 18k$/year. Times 16 is 288k$/year; and someone consuming 288k$/year is a decent approximation of a "millionaire" lifestyle.
And if all of the power comes from carbon-free solar and similar (even planes use batteries; batteries are chemical fuel cells that can be easily "recycled" with electricity), it is possible (but difficult) that the ecological impact won't be much off modern day society.
The world is pulling off 2%-3% annual growth rate on a 1% growth in population; so closer to 1%-2% GDP per capital growth rate than the above 2.5%. To hit 16x wealthier per person, you'd need 186 years of 1.5% per capita growth.
That end state -- where most everyone on the planet lives at the wealth scales of a modern western millionaire -- is very difficult, but not completely implausible.
One of the reasons it isn't implausible is that the industrialized world already did it. Via economic growth, they "turned everyone into millionaires".
They took the age-old truth that almost everyone lives at the edge of starvation -- the truth of 1600 AD -- and made everyone on average 100x of times richer. By many measures, the average person in an industrialized or post-industrial country lives at a standard of living exceeding the richest 0.1% of people in 1600 AD.
And the same thing is happening in many parts of the world. There are areas going from starvation level poverty being the norm to it being an extreme exception in a matter of generations.
There remains the real, difficult problems of not destroying the planet while doing this.
But that is why economics talks about growth, and considers it highly valuable, and not growing to be avoided. Societies that grew developed the science of modern economics, and the results on the well being of the members of society are huge.
