Even with the 80% cap, it is still possible to score 100% in some subjects, and universities could focus on the grades in key subjects rather than just the overall average.
Do you think this grading system would reduce stress and encourage career planning?
Do students want this? No. They will want to distinguish themselves from their peers and trying to cap their performance prevents that.
Do institutions want this? No. They also want distinguishing power.
Do parents want this? Probably least of all. They want their kids distinguished and caps won't work.
No individual participant has the desire for this. You also can't talk them into the desire by doing things like saying "but nobody likes stress", because you don't control what is motivating people by how you squint at their motivations. Their motivations are their motivations.
As a result, even if you were King of the World and pushed the Magic Button that instantly and completely enforced this everywhere, it would immediately decay as every participant in the system worked around it, rather than for it.
I type this to encourage you to think in this way, because it is relevant to doing good engineering in general. Solutions to problems must also be implementable, and something the people with the problems actually want. And what they "should" want is pretty much irrelevant; what they do want and what the actually do are what matters.