if you are planning to leave soon and will have no stake in the future of the company, there isn't much incentive to take it into account when voting on decisions.

You're comparing that unfavorably to today, where 1 person who may be leaving is making that decision compared to a group of people who are far less likely to all be leaving. My proposal is an improvement on the current state.

why let an unaccountable person have influence over crucial decisions?

They're accountable to the workers that voted then in to that position and can vote them out. This is compared to what accountability today?

The natural response there I think is to want to heavily favor new hires that you are personally connected to; where you already have an interest in their enrichment, and who have external reasons to be invested in your interests and the interests of the company, which is why I mention nepotism and think it would be a bigger problem.

With decision making being democratised across employees who have a vested interest in the success of the company, this would be a smaller problem than today where one unaccountable manager can make a unilateral decision.

(the interests of the workers are the interests of the nation (Why would they be?

Because the vast majority of the population works for a living. Handing the wealth and reins of power to unproductive owners as we do today is massively detrimental to the economy. People would be democratically acting in their own self-interest.

With a higher baseline and less desperation it would be more feasible to form worker collectives to begin with, without a need for state mandates enforcing them to be operated in a particular way.

In a more equitable society, there would be less motivation to collectivise, but whatever gets the job done. To reach that point, we'd need to engage in some pretty fundamental restructuring of the economy though - I'm not sure why we wouldn't just keep on trucking toward better outcomes rather than stopping half way.