If you consider an ultra bear scenario where BTC loses 90% off it's ATH ~$1990 and then consider that BTC has lost about half of its marketshare going from 80% to 40% (and sometimes less) of the total crypto market cap then realistically the ultimate dip could test $1000. That being said, if such a scenario played out the market would have to be suffering from some extreme dysfunction or shock (like a big hack similar to Mt. Gox on a relative scale) and given other factors such as the potential for institutional investors buying large amounts and Bitcoin halving on June 2, 2020 a bear market reversal would logically coincide with expectations of reduced coin emissions.
While other coins could theoretically displace Bitcoin what we really haven't seen are real network effects from other coins and the corresponding infrastructure built around them. Simply forking Bitcoin is the easy part, but recruiting users and building actual systems around a fork is hard and if the economic incentives aren't there then those coins likely will struggle to survive.
This new coin "eco" from Garrett Camp, co-founder of Uber, looks interesting, but when you read the white paper, it's just another fantasy that will take far more research and effort than the founders realize. The great thing about Bitcoin is that it is "as real" it gets when you think about money or an asset whereas other coins like ADA, XLM, and NEO are still very early, and feel like vaporware. An ideal portfolio would allocate a percentage to a number of different coins but given the possibility that the markets (including equities, hard assets, etc.) will get soft for an extended period of time it will pay to be patient.