Private yachts and jets aren't good things though. Neither are mansions. Why should we be happy about people building them when they could be doing something else?
You might disagree, but I am sure that you can think of examples of spending and employment that make people worse off even if they improve our national standing on basic economic indicators.
To echo your words: We have an entire generation of people who grew up being taught that any increase to GDP is an objective good; that GDP is an objective measure of our collective success as a country; that the most important goal for any political leader should be increasing this number a few percentage points over the previous year. Never mind the environment, never mind social norms, never mind community, never mind human health, never mind human dignity, never mind that it's extremely difficult to understand what GDP actually means, particularly in a heavily financialized economy.
The point of this litany isn't to dismiss economics out of hand or to justify illiteracy or lack of curiosity. It's that we should care about things beyond economic growth, and that we should understand what economic growth actually comprises and whether that is something worthwhile. We have an array of values against which economic growth ought to be judged, and many of them we cannot coherently reduce to another number in an enormous cost-benefit analysis. You can't just say that GDP is growing or that jobs have been created and celebrate this as an achievement - we need to actually understand how life is changing.
Moreover, in certain respects there really is only so much wealth in the world - from non-renewable natural resources that exist on this planet in finite quantities (e.g. fossil fuels, heavy metals), to natural processes with a finite capacity (e.g. hydropower, the water cycle), to limitations of space (e.g. there is only so much coastline that we can develop).
The main impact of billionaires would appear to be that they hold back median income.
Of course, if someone's understanding of economics starts and ends with a supply-and-demand graph, you wind up with posts like vcrew's.