You're definitely right that you picked apart their argument because ackshually there will always be a richest person. But clearly the sentiment is that someone shouldn't get excessive wealth past their threshold.

How do we define excessive wealth and how do we limit it? Well there are lots and lots of proposals I would suggest reading up on some (you can Google that question to get 10 op eds that suggest 20 different solutions). I wouldn't mind defining it as a certain percentage higher than the median wealth of the country. It would be funny to give Gabe Newell a "you won capitalism" trophy and taking excess wealth he gains.

As for motivation. It's a much murkier subject than you imply. In an economy where the state takes every penny of a successful business's wealth, yeah it makes sense that there's no motivation to make a successful business. But if one could still get rich off of running a business (just not god-tier level wealth) I'm sure there would be plenty of motivation. And hell, if we give them prestige like we do now there's tons of people who do what they do just for the fame with no profit. There's tons of evidence that people aren't purely motivated by the infinite profit of business people all over the world work their asses off in jobs they enjoy or respect that will never pay them Gabe Newell bucks.