That's naively optimistic. Circumstances can change, and relationships can sour; often, it can happen through no fault of your own. It could a simple a disagreement in the face of mounting stress, somebody needing a quick exit due to life-changing events like an illness, or even just a deadlock over a key decision. Startups are stressful in general, and they usually move at high speed: things change, often quickly.
Shareholders' agreements and other contracts (such as those with early consultants and employees, like Jeremy Guillory with Cruise) are meant to protect against this uncertainty. Simply ensuring that there's a clear exit strategy for the founders can ease possible tensions before they arise just through the knowledge that they exist. And if things do go south, it ensures that an exit is as painless as possible for all involved.
If you assume things are going to go smoothly, at some point, you're going to be in for a very painful surprise. You should always figure out an exit strategy. Anything less is just dancing on the edge of the volcano.
Making decisions that optimize outcomes for current founders over ex-founders is not the same as assuming there won't be founder disputes or exits. I just think it creates better incentives.
I might have misread your intention then, but there shouldn't be a tradeoff between the two at all. If I understand your point correctly, then your agreements would prioritize founders after an exit by eliminating the likelihood of an equity dispute later on but not at the expense of the person who exited at the time of their exit. If that makes sense. In general, shareholders' agreements are all about fair dealing and codifying the nature of your relationships and potential business interactions with one another. They'll cover all sorts of different scenarios that go well beyond buy-sell provisions. Here's a very thorough sample from the ABA [0] for a closely-held corporation. Your attorney would work with you to tailor an agreement to your needs, obviously.
In the end, you're just buying out their equity in the company at the time of the sale. Nobody really comes out "ahead." But because nobody really knows how they could arrive at that situation, it's important to ensure that your likely contingencies are covered and that the actual process is clearly defined. For instance, you want to make damned certain that someone can't come back for a second bite at the apple after the fact. Even if their claim is specious, it will still derail a deal.
Shareholders' agreements and other contracts (such as those with early consultants and employees, like Jeremy Guillory with Cruise) are meant to protect against this uncertainty. Simply ensuring that there's a clear exit strategy for the founders can ease possible tensions before they arise just through the knowledge that they exist. And if things do go south, it ensures that an exit is as painless as possible for all involved.
If you assume things are going to go smoothly, at some point, you're going to be in for a very painful surprise. You should always figure out an exit strategy. Anything less is just dancing on the edge of the volcano.