Not against diversifying but people should know how to do it before getting themselves involves in what they don't know. The wealthy give advice about diversifying but they don't tell you that, majority of their capital are in fewer assets and then the rest scattered around other investments.
That’s true and that’s where new investors make mistakes, they will want to diversify because they are advised to do so, therefore, once they have money, they don’t look for further advices or consult experienced people, they just go in straight and that’s where most of get it wrong which do lead to losing their capital especially if they invested in something risky.
Prioritize having one main investment/asset that you put more time,/money in instead of scattering your money around as that won't give you focus as a long term investors. One thing you'll notice about the wealthy individuals is that they made majority of their wealth from one asset or sector then have smaller accumulations in other things.
It'd easier when you have a goal that you can put all your attention on achieving than when you have different things taking your attention (funds). I'm not saying you shouldn't diversity but don't be in a rush to do that when you aren't actually wealth yet because diversifying should be protecting your wealth more, that's how the rich play the game.
Having one single source is not bad, but I think it is not bad to diversify either if you are yet to get a single direction you will be making money from, but if you got one main investment clear and knows more about it, you can focus on that and make good money from that investment instead of diversifying in wrong investments.