> A CEO might work 90 hours, but it isn’t 90 hours on the assembly line. It’s 90 hours building all parts of the business.
More like 90 hours "supervising" the people actually building all parts of the business (or, more commonly, supervising one or more intermediate layers of supervisors who then supervise the people actually building all parts of the business). And that's taking the implication of 90 hour CEO workweeks being even common (let alone representative of most CEOs) at face value.
More importantly, the Marxist response didn't/doesn't revolve around the nature of the work; rather, it revolves around the ownership of the profits from that work. Specifically, the Marxist argument is that the workers are the ones actually creating the vast majority of the company's value, yet do not typically receive commensurate compensation (i.e. wages + benefits + ownership stake). Ownership stake in particular is the main point of contention: the difference between "value created by this worker" minus "compensation paid to this worker" (which is necessarily a number greater than zero if executives/managers are getting salaries) would in any honest accounting be an investment - yet rarely do workers receive shares of the companies they build, and when they do it's typically a small fraction of what they "paid in". Likewise, employees rarely have any control over who their executives and supervisors even are; if their CEO spends 90 hours a week "working" on golf courses and in fancy restaurants with CEOs of other companies, employees rarely have any ability to fire that deadbeat and hire a replacement who actually provides value to their company.
That, if you remember from your Marxism classes, is the estrangement of workers from their labor. They work, but are denied commensurate ownership/control over the results of that work. This is what cooperatives and unions seek to resolve "within" a capitalist system - whether by creating companies which do give workers commensurate ownership or by pressuring existing companies to address their employees' needs/wants.
> Modern history is filled with capitalists complaining about their lazy children.
History in general is filled with New Money's descendants becoming Old Money. New Money complaining about said descendants becoming Old Money ain't exactly unique to capitalism.
> I can’t imagine a serious conservative political party called the Leisure Party.
Nor can I imagine one called the Racism Party, yet my country manages to have multiple parties for whom such a name would be quite apt.