The price volatility is way more expensive than transaction cost, if you have billions of dollars.
Anyways, there are plenty of stablecoins if that's your thing and you can use Bitcoin (the network) for the rails only.
Also, stablecoins staying “stable” is a huge risk you are taking on if you use them.
Again, if you have billions of dollars, you are going to be doing a lot more risk analysis and want a lot less uncertainty than you are going to get with any sort of cryptocurrency that exists today.