sashbar I get what you're saying and agree with it, but I think you're presenting it slightly the wrong way; and in a form that favours the intermediate over the beginner.
I think a beginner has to learn with a broad approach, because unless they have a very singular very focused interest chances are that they are going to want to do a bit of everything at some stage. In addition learning different things gives you more potential tools to work with.
The more tools - the more experiences - the more difference you have the more potential you add. The more you can adapt, even within a niche having experience outside of it will give new possibilities or just make you look at things differently - or be ready for that time when something within your niche does something different.
Yes the more you learn the more you realise you don't know. For most people they don't just learn and learn and learn - they reach a point. A maximum. Typically this will be a point at which they can perform their chosen interest area(s) and get results that they find pleasing. Those of us online (esp active ones) tend to represent those who might find that we don't reach that point as readily or as fully - we see a little further because we are the "keen" ones; so its harder to imagine someone who reaches a "yeah this is enough" point so readily.
We are going to forget things - heck I would say that pros are in even worse a position than amateurs. A pro is likely (in most fields) to be doing the same sort of thing every day. They will have a style and a working formula and will produce to that style. All day - every day.
Now sure most will have different niches and some will cover several so its not always identical; but they key is that for every season of style that they have (and that might be years or months long) they in essence shoot within a niche. So they've a great potential to get into that niche and not come out.
An amateur - or a pro with more creative freedom than most are allowed - has the bonus that they more so choose their niche as they wish. If they feel like change they just change -a pro with a long term style has to seriously consider change (esp if its significant) because that could lose them their market corner.
In all this though we are talking of ideals; ideal concept people (pro - amateur) and in reality they don't exist. Reality will take bits of these ideals and will come close or far to them.
Really what it comes down to is the individual - and I'd always say the best advice you can give them is to keep that broad view - to keep learning - to keep experimenting - enjoying and doing their hobby. Yep they are going to forget stuff - if they are AWARE of that they can better prepare themselves. They can refresh - they can try something new etc....