Your title says a $100k by 2021 yet the content says it will hit $500k, it's contradictory. Well, it's possible but I think we have a long way on that mark and within weeks we are hanging on the purgatory of $9k either we go hell or heaven the next day/s or week/s.
you can see the $100k
https://coinpedia.org/news/max-keiser-predicts-bitcoin-at-500k/S2F Model Predicts Bitcoin Price Accurately-Analyst PlanB
The S2F model considers BTC as a commodity like gold or silver. It considers the existing supply of the cryptocurrency and the mined cryptocurrency and predicts the BTC price. According to this model, the BTC price might hit $100,000 by the end of 2021.
My take on this, I think bitcoin will hit at that price but I don't think it would happen next year.
This kind of prediction is too bullish and I just respect it that way as I don't want to call it unrealistic since everyone of us can make our own prediction based on different factors.