it's okay if you trust ARB more than Bitcoin, I personally don't really like risk, so I prefer to keep holding Bitcoin instead of inflaming Bitcoin just to buy ARB, because I believe if a bullrun occurs, then the price of Bitcoin will increase very high and of course the price of Bitcoin will create a new ATH price (I can get a big profit too from Bitcoin).
You do not need to sell bitcoin for ARB, also buying altcoins is just gambling, but I think it is gambling that is really worth it. Assuming you have 70% allocation for bitcoin while only 30% for altcoins. Out of the 30% for altcoins, you can choose some percentage for ARB and just invest. But people should be mindful for now that if bitcoin go to $20000, altcoins would fall as expected and the fall would be very massive.