If you don't take into account what is being produced and why, you are left with a line of tautological reasoning that more productivity is good because more productivity is good.
The reason this is idiotic because there are a lot of ways to maximize productivity where the vast majority of people do not benefit. Not only that, but there is absolutely no reason that maximizing the productiveness of an economy results in maximum utility (in the philosophical sense).
Even if you could demonstrate that economic productivity was positively correlated with (philosophical) utility, you'd have absolutely no guarantee that utility would be distributed equitably. You could literally have 1 super happy person with everyone else on the planet enslaved in perpetual (productive) agony.
That's even before you get into the fact that maximizing production now probably ends up creating a hellscape of a planet tomorrow.
Of course you could argue that you can account for all of this using negative externalities. In that case you might as well tax capital gains to account for the negative externalities. I suppose you could tax something else, like lifetime earnings, but the point is I feel comfortable making the claim that not taxing capital gains is idiotic.