BTC can touch $318K by December 2021. What do you think? Is this really possible?
It's certainly possible. Not sure how likely it is, but possible, yes.
Objections to this are sometimes based on a misunderstanding of exponential growth or a misunderstanding of marketcap.
Below is a screengrab from CMC of recent BTC price progression - on a log scale. The objection that "it's only gone from $0 to $20k so far, therefore $300k is out of reach" overlooks the fact that this is a single order of magnitude change, which we've already seen many times before. Granted the progression gets harder each time, but there's no indication that we're anywhere near a ceiling yet.
Secondly the argument that it's unrealistic because it means a stratospheric marketcap - marketcap is misleading, it's just the current price based on current trades. It certainly doesn't mean that's the total value of the coin. BTC cap is currently $300b. This doesn't mean that all BTC could be sold right now for $300b, because each sale causes the price to change. Marketcap is just a rough guide and shouldn't be taken at face value, and shouldn't be taken as a limiting factor.