There's a difference between "selling" and "selling out". Selling your company does not mean necessarily mean "selling out". New ownership often means new control, new principles, new directions, often driven by pure money-motive, it isn't always. Berkshire Hathaway is famously hands-off with its acquisitions, for example.
The "pro" side will typically argue that a given sale is just changing ownership structure but not fundamentally auctioning off their values/trust/etc.
Abandon can mean relinquishing of control. It would depend on your employees/clients, but given a large enough sample size, I'm sure one party might feel cheated when power changes hands.
That said, ownership gives you that power. The power to relinquish it. It seems perfectly ethical/moral to me.
The world is not a perfectly efficient free market of isolated transactions.
Does Bill Gates have a positive moral duty to all those "stakeholders" employed at MS, until the day he is carted onto his deathbed? Obviously not. What would that duty actually mean in concrete terms? I have no idea. In fact, at some point, trying to hold on and control the company is in the disinterest of stakeholders, as Gates' personal interest and abilities wane.
TFA is pretty vague too. I do not sell anything particularly wrong with selling a company at 10% of what you could have gotten several years later. A "measly" 2-3 million dollars in a bank account buys you a lot of freedom. Diminishing marginal returns means that another 10 or 20 or even 30 million buys you less happiness than you might think.