In investing, holding is generally safer and more stable than trading. However, that doesn't mean every asset or crypto will make you money just by holding it.
Most altcoins are largely speculative, and many are simply junk. Holding them for the long term is a bad idea. Only Bitcoin is a crypto worth holding for the long term.
Well, investment means holding. If you trade the coins, you're not investing the coins.

Anyway, it is actually what I said before. Holding altcoins aren't the same as holding Bitcoin. Holding random altcoins for a long term possibly ends up with losses only. Many altcoins turn to be dead coins and they have no value anymore after few years.
In my opinion, proper diversification means spreading money across assets with little or no correlation, not just buying multiple asset that are closely correlated. If they all move in the same direction as the crypto market, then you are only diversifying by numbers, not actually reducing your risk.
Do you mean diversification must be in different fields?
I think there is no problem to diversify in other crypto coins. However, we must ensure to diversify in trusted coins (BNB, XRP, ETH, TRX, etc).
Diversification in other assets such as Gold or landed property is also good idea. But we must ensure that we have good understanding about those assets as well.
Investing always comes with risk, and whether you are new or experienced, diversification is still essential. But as I said, diversify across assets that are not closely correlated with each other.
Sure, any investment has its own risk. And diversification may also have the risk. I'm not really sure what your point is. But I will always suggest anyone to diversify into something you really understand.