If you are right and Bitcoin loses over 80% of its value in 2021 then it will be all red on the crypto price websites. In that scenario many smaller coins and token will probably not even survive and others will see a huge fall in price.
Many investors will cut their losses reluctantly when a correction happens and many will simply face financial ruin when the crash happens when panic selling across the world sets in because they bought in to crypto at high prices.
it won't crash anytime soon but some corrections may occur like 30% and max 40%, which mean around $24k - $28k.
The real crash will take place after bitcoin reach the peak probably by Q4 2021 if history repeats itself. Then bitcoin price will gradually lose its value to 80%-90% in 2022.