Why? In an hypothetical world where getting a loan on an asset is impossible (or taxed the same as realizing the gains), you still don't get taxed on unrealized gains. You can leave your stock alone and you aren't forced to sell anything.
Of course if you decide that now that you are worth a billion you must live like a billionaire, then yes, you will have to sell stock, reduce your influence in the company and pay tax on the gains.
I don't see any problem with this? It offers a way for the stock owner to choose if they want to use the stock as power (don't touch it) or as cash (sell it), only taxing you when you opt for the later.
edit: I realised I might have misread your post as defending the system allowing one to use unrealized gains to back a loan, hence enabling the buy/borrow/die loophole, when you are in fact defending against taxing unrealized gains. To me the obvious fix is to prevent those loans as discussed above: force people to choose how they want to use their assets, if they choose to use them to live like kings then they must pay tax.