You know the market for NGL is much broader than the market for petrochemicals and can actually diverge. The petrochemical market, e.g. ethylene/PE/PET/PS and PP, is not necessarily affected until such time as the manufacture of widgets slows due to economic downturns. So not much effect for ethane and propane. The problem today is propane prices are depressed due to lack of access to markets. Hence the reason for the Ripley terminal, and the PPL and IPL PP projects. I doubt ethane and propane prices have moved, or will move in the short term.Why would the market for NGL/petrochemicals NOT be affected by a downturn in global markets?
The bigger issue today for the likes of GEI, PPL, KEY and IPL today is the drop in pricing for the heavier gas liquids used to provide diluent for bitumen blending. The collapse in oil prices will directly impact the prices for pentanes plus and I imagine butane as well. Oilsands producers simply can't pay the same prices for diluent and to the extent some oilsands production gets shut in, demand for diluent will go down. CVE has already said they are cutting back on crude by rail.
So one cannot lump petrochemical demand and thus the feedstock for petrochemicals into the same bucket as NGLs used primarily for diluent. They are only loosely correlated. Building the PP plants can be highly profitable for the most part, distinctly separate from the NGL processing business.
Added: Link to Nov 2018 NGL Study https://ceri.ca/assets/files/Study_176_Full_Report.pdf