So, what happens when bitcoin and other cryptocurrencies become so highly valued that we can't pay enough cash
to cover the taxes? Like, what if I have $20,000 in taxes to pay but I haven't the cash to pay it? But I have it in crypto?

I guess we would have to cash-out some of our bitcoin or other cryptocurrency in order to pay
the tax. And then we'll have to pay tax on that cash-out.
 
Maybe to keep it simple, the IRS would simply accept bitcoin or other cryptocurrencies for paying taxes.