That might help, but even without stock incentives, most managers promote by appearances, credentials, diversity, connections, and recent performance. Indeed, with frequent changes in jobs and duties, it is often hard to accurately and fairly assess long term job performance. And genuine merit and good performance by an employee can be threatening to superiors.
Long term stock price is the measure of long term performance.
Let us say a new President of the company is hired in 2024.
As you say we have no idea whether they will last just six months or twenty years.
However, the long term compensation simply says that there will be deferred payments (details in employment contract) that will be based on the stock price in 2034.
Obviously those payments cannot be made until 2034.
That would motivate the President to think long term.