Bitcoin is profitable, altcoin are profitable too. But I just need to convince myself that investing in bitcoin is safer than altcoins. Diversification may be good – but risk tolerance must still be considered because each of us has a different tolerance. It doesn't have to be an early investor - I mean returns can always be expected even if they just started investing today.
You are right that returns can always be expected even if we start investing today but it won't be like early investors with bitcoin. At least not in another 10 years. It will be totally unrealistic to think bitcoin will make another 50,000x from here in next 10 years. Its market cap was small in its early stage which is why we have seen a higher ROI though 10x from here will make a market cap that will be bigger than its previous 10 years combined. Some people came to the industry to make 100X with 10K so new altcoin is best suited for them as it can give you a SOL or Matic like return in the future.