Well, if we switch to natgas, the supply of both will increase. Sure, some of the "multi-carbon" species of natgas will be siphoned off as chemical feedstocks (ethylene, propylene, possibly some butylene), but I seriously doubt the chem companies will take the entire supply.
What's the typical ratio of C to C2 and C3 in shale gas, anyhow?
No such thing as typical. Just like in traditional fields, the ratios vary all over the place.
Eagle Ford in Texas is very wet, an oil formation on one side with a gas formation on the other and blended in between.
Haynesville is very dry gas with almost no natural gas liquids at all.
That is why today Haynesville drilling has been falling off while Eagle Ford and other wet formations are booming. The liquids have become the money making target for much new drilling and the Natural Gas is a by-product.