Personally I would go for a 100% BTC focused portfolio IF I had to choose between the two, but I believe that altcoins have higher growth potential than BTC just because we haven't seen any serious institutional funds pouring into them. Who knows what heights can them achieve if that would happen? BTC is the less risky choice though if the potential reward is somehow more conservative.
As you say, if you are a safe and low-risk person, invest 100% in bitcoin, and if you are high risk and big reward put your money in altcoin, depending on the benefit. The profit you want and the risk you can bear, in the altcoin market is often unstable so you need to watch closely and focus, for me, I will choose both, one for the long term and one for the short term.
While what you are saying is right, many traders do not really have an idea about how to calculate their risk and in such cases it is always better to play it safe and take the lesser of the two different risks, in this case that will be to invest everything in bitcoin because as we know bitcoin is by far the best and most solid coin in the market that no matter what happens it is never going to disappear unlike the majority of altcoins.
Some investors may believe that this strategy is simply too conservative for their tastes but whenever you are investing it is better to take the more conservative approach first and then once you gain experience you can take more risks.