Both bitcoin and altcoins are going to see increase during 2021, but don't get carried by the hype train, they will still fluctuate, but something almost certain is that they are going to become stronger while the fiat currencies are going to get weaker.
Which fiat currency will weaken? because in this world there are so many fiat currencies and some of them are also very expensive and their value always lasts, so I am a little confused if you say Bitcoin and Altcoins will become very strong and other currencies will weaken, even though everyone who buys Bitcoin and Altcoins always uses fiat money at the start when making transactions via banks into crypto accounts.