> As a student, the only thing the next institution will see is GPA, school, major. Roughly in that order. If the cost of not getting an A is exclusion from future opportunities- then students will reject exclusion by taking easier classes or cheating.

That's not the cost of not getting an A, it's the cost of appearing to underperform compared to too many of your peers. Which is directly tied to how many of them cheat. If not enough cheaters got an A then the cost would no longer be tied to not getting an A, it would be tied to whatever metric they appeared to outperform you on.