You can say that 200 btc will be enough for you in future. but it can't. because you don't know what will be bitcoin price in future.
it may go up or may go down also. so you can't predict about 20 years later right now. may be bitcoin price to to the price of 1$ per bitcoin. then?
this is like gambling. you can buy 200 btc for future and take that risk. you may get lots of profit that time and may lost all your money also.
that is why it is always better to diversify. it is never a good idea to just put all your eggs in one basket and then sit around and dream about a rich future. you should constantly work and add to the pile and also invest it to multiply the initial amount too.