I assume that since the 50+ markers are included, you can (regularly?) see incomes get that high.
False. It's very rare, unless your group is perhaps new and/or plays in a very odd fashion.
My question is how? It seems that if income goes up too high, shares get sold quickly (thus hastening the end of the game).
Is there a playing style where you see those high incomes?
Probably, just not a good one. I've seen one company one time get to around 50. I've played ~40 games. Normally low 40's are the highest you'll see, more normal are low 30's for some companies, while others finish in the ~20 range.