That is why you file 83(b). Because 83(b) says that you want taxes evaluated to be on the date the shares is granted to you, not the date it is vested. Because when it is granted to you, the share is worth pennies, but later as say 4 years later, when it is vested, it could be worth millions on paper.
Regardless of whether you file 83(b) or not, if you sell the shares, you still have to pay the tax on capital gains. However, if you can sell your shares, at least it means that you had a liquidity event, not paper millions.
My understanding is that for capital gains it is only triggered when you sell. (Income tax is triggered when you receive an asset, and taxed based the value of the asset of at the time you received it). So my understanding is that the 83(b) basically says you are electing to make the shares you received now to as the trigger time for you receive the asset (and taxed as if you received current value), instead of receiving the value asset as when you are vested (which time the value is a lot higher).
An example would be: today: you receive $100 worth of shares. Income is taxed at 30%. (For simplicity, let’s say you didn’t pay anything for those shares.) Suppose the shares value 10x for next 4 years, and you sell in year 5.
If you did file 83(b), the $100 is your income today. so you should pay 30% income tax on $100, or $30. And suppose five years later (10x increase per year). the shares are worth $10,000,000. So you sold $10M, you’ll have to pay capital gains tax on $10,000,000 - $100 * 15% long term capital gains tax.
Suppose you didn’t file 83(b). On first year, 25% of your shares vested, shares value increased by 10x, so you have to pay income taxes on what you received = $250 and in second year, share values increased by 10x * 10x, so you’ll have you pay income tax on $2500, and so on. By the fourth year, you’ll have to pay income tax on $250,000 income of shares that are vested.
So on fifth year, when you sell, you pay the capital gains tax on ($10 M - $250K - $25000 - $2500 - $250) * 15%.
Not a huge difference if your stock indeed ended up being worth $10M, but of course, the worse case scenario could be that you never get a chance to sell, so still paid all the taxes on the shares’s par value as they are vested.