My opinion is that in time Bitcoin market share will keep reducing, as many other coins and tokens and platforms will keep surging.
In the long term, the blockchain technology is very disruptive. Bitcoin will more likely compete with Gold, Ethereum will provide a backbone for many uses.
So I am not really sure a collapse is incoming, it depends if a real bubble will quickly form.
Do you understand that the market of gold is one of the biggest around the world, if bitcoin becomes big enough to rival gold I would consider that a huge success, also if the market of bitcoin becomes so big that means that the market of any coin trying to surpassing it will need to be even bigger, something I do not know if it is possible in your scenario.
No I dont think so. Fall in the price I think is for the time being and it will grow again in the near future. We just need patience to hold our coins for some more time. No doubt 2018 gave deficits to many people but those who hold their coins will make good profits in the future. Dont be panic about the price in this day and be optimistic about the bright future.