I agree with you, its better to be strong in one asset (bitcoin), than to have so many weak investments. If I have build my bitcoin portfolio at least to maybe up to 5 years of accumulation I can now start think of diversifying but this might be too early in some case especially when i have not accumulated enough bitcoin, I always wanted to have at least 10 bitcoin before I considered any other investment, but you have also told me I should be prepared for a case where my investment doesn't give any return, and till now I've not actually understood the lesson you are trying to give me in respect to this, I'm building up myself in terms of cash too and I'm still consistent with buying bitcoin with DCA.
Once you can position yourself well in Bitcoin investment, you can start diversifying. But this diversification in altcoins may be more confusing because there are so many altcoins that we don't know which altcoins can rise. Moreover, it looks like altcoin season is about to start so if you have difficulty choosing an altcoin, you might have difficulty making a profit.
I consider JJG'S advice that after building up your Bitcoin investment portfolio to a certain point of considering diversification, that it is advisable to diversify in to real life investment such as selling commodities, real estate or land, investing in altcoin is more like gambling than investment.