The cause of BTC's decline is not Bitcoin Cash. The BTC failures as money are the result of poor economic decisions made by Bitcoin Core (which considers it "digital gold"). That BCH grows in adoption and price (as long as it does) as a result of its qualities (which better meet the needs of the market) is not bad for the ecosystem. Maybe that affects BTC (especially if the price gap closes much more), but that is part of the market process.