I'm an example of a not-even-particularly-early employee that joined a startup at Series A, exercised the majority of my options almost five years ago, and have been holding on to the now-public shares with the intent of taking advantage of QSBS. I have literally watched the stock go up and down without pulling the trigger because of the promised tax breaks. Pulling the rug from underneath me is not what I would call ethical. There are many other early employees in my cohort doing the same.

Many people join companies early enough to be compensated in equity, not options and also don't have the title/requirement of "co founder". People also early exercise.

Lots of people still get options that expire pretty soon after leaving a company, right? So unless you want to be stuck at one company, you have to exercise.