You own a company so valuable that the tax on selling it shares puts you in serious trouble, but you still need to a start "a job"? A Job that requires you to change your nationality/tax residency to a non EU-state?

Edit: Maybe I should give an example: Lets say you build up a company, your shares are worth ~100k, while you payed yourself a living salary of ~2k so you could pay rent and buy groceries but not much more, especially no savings. Now you get on offer to work in the US for 180k/a, you sigh "finally" and just want to move, but the German wants 30k Taxes on your unrealized ~100k capital gain before your leave - Is this the kind of situation you are referring to?

Pretty much. I just have a symbolic 5% stake in the company and working there has certainly not made me rich. I'm still early in my career so I don't have a ton of savings. It wouldn't cause "serious trouble" but paying tens of thousands in taxes on top of the cost and hassle involved in moving overseas is still a significant burden and certainly a case of the regulation missing its mark.

how about not taxing money you dont have?