I know that people like to comfort themselves by looking at the past - but the past is no guarantee of the future. Lots has changed since 2015, not least the emergence of some real competitors to bitcoin, and bitcoin merchant adoption going into reverse.
In relation to Bitcoin's historical trend, I wouldn't worry about the emergence of competitors. Competitors have been emerging for years, and nothing has ever changed. Look at ETH and DASH, for instance. Their historically bullish trends vs. BTC haven't threatened BTC's gains.
And that's because of the fundamental relationship between BTC/fiat and BTC/altcoin markets. Altcoins are actually very bullish for BTC price! The existence of altcoins continually dries up BTC supply that would otherwise provide downwards pressure on BTC/fiat markets. Just like masternode coins and ICOs remove market liquidity and drives the price of their underlying coins upwards, the same is true of altcoins vs. BTC. They remove market supply on the BTC/fiat markets, driving the price upwards.
Conversely, if you look at positive price correlations between BTC and altcoins, strong Bitcoin bear trends (with occasional exceptions) massacre altcoins, including the top altcoins like ETH. So altcoiners should really be careful when they predict and hope for Bitcoin's demise. Altcoiners should hope for BTC sideways and consolidation. It's during sideways that all the attention moves to altcoins.