I agree with you, investing in Bitcoin is not a bad thing in fact it will be very profitable if we can manage it well. Even though it fluctuates, the risk ratio when you invest in fiat is almost the same. Bitcoin gives us the freedom to manage it. When the market goes down continuously many people panic, even though this can be a profit opportunity to reap a lot of profits when the market stabilizes. Always take a positive point in the steps you choose. If there is a failure and you experience a loss, take it as a lesson and don't repeat the same mistakes.

I think that advice still applies, and applies to any kind of investing, not just Bitcoin, especially if the investing is done by a newbie with little understanding of the market they are investing in. It's better to be safe than sorry, in my opinion.

You seem not to understand the OP point because he was talking about Bitcoin investment and the main advice giving investors is to only invest the amount they can afford to lose.
Of which the statement seems not to be valid lately because Bitcoin is safe haven than fiat currency. After all, enormous fiat will be printed this year and the debt rate is increasing.

Investing the amount you know you can loose is a good idea because tend to live in shock for a long time after the loss and it's also nice when you get paid as an investor from IRS😁 as they are paid in USDT when they hold $IRS instead of knocking on your door collecting taxes

Thats true, regardless how good the investment is (in general, not only Bitcoin) the basic rule should be applied. However it depends actually for each investors because if you educate yourself first and aware of the risk then its up to you whether you'll follow this specific rule when investing or follow your gut.

I think for investors who already know the history of Bitcoin and experience the ups and down of the market, taking risk to invest on Bitcoin is much wiser than saving fiat without gaining anything.

I also thought of such a thing. maybe a good strategy is to insert a few percent of the salary or income that we have as savings with a note that the money does not matter reducing the amount, because we do not know when the price of bitcoin dumps, or pumps. well, we're just waiting for the moment for that. however, this is also just my opinion if you want to have a large amount of bitcoin but with a movement that does not reduce significant funds.

Someone said before that bitcoin as a safe haven but I do not know if what are their basis to say that btc was really a safe heaven because in my opinion btc is way to risky to consider as one due to its volatile nature which the price goes up and down or maybe they think btc was a safe haven because it is immune to inflation and the money that they put on btc wont get devalued overtime (disregard the fluctuations). 

For them, it is better if they can put much money on btc than to put it in banks because it will only get eaten up but to us we are just following the famous rule in investing which was to invest only what we can afford to lose.

Although many people believe that bitcoin is not risky and is a good investment option, it is important to only invest an amount that you can be fine with if lost because bitcoin is still not legalised and has a good chance of going down.

^ If the 5% is the amount that you can afford, that good enough.
This advice "Only invest an amount you can afford to lose" is something that makes me shudder whenever I hear it.
Why? Because it is really not good advice for me. Of course,  it is important to remember that all investments involve risks, but should you just put money into something you're willing to lose? Please, please, please. The same as saying "don't put money into it or do not invest!". I don't want to risk losing the money I have put in. Taking measured risks is something I am willing to do.