Investing in Bitcoin is not a form of gambling. So I disagree with your statement that you should invest in Bitcoin only as much money as you can afford to lose in the long term.
But here, I will disagree with your statement because the phrase as much money as you can afford to lose is used when investing in Bitcoin not only because it is a gambling but also because Bitcoin has many other risk factors such as it is a highly volatile asset and at the same time since Bitcoin is a virtual currency, there are other risks, such as wallet being compromised or the platform where you are holding Bitcoin getting crashed although bitcoin shouldn't be hold on centralized platform.
When you put money in a bank you can lose your money because the bank can go bankrupt and such things happen. When you invest in gold it can be stolen. When you invest in a house property it can be destroyed in an earthquake. There is no investment in the world that is risk-free. However, you have to use your intelligence to deal with the risk. Why would you keep your Bitcoin on a centralized exchange? You know that your Bitcoin is not safe there. Why don't you keep your personal wallet private key carefully where the key is the only life of your Bitcoin? All investments have to be taken care of and managed smartly, then it will not be difficult to protect them.
So you think investing in bitcoin and investing in the other assets like property or gold, or a bank have the same risk? If you think so, then I would say you are wrong because in real life if your gold is stolen or your property is damaged in a natural disaster, then even you will have the partial amount of your asset, but if you are a victim of wallet compromise, then you will not get any of your bitcoins once the transaction is completed. Even if we leave aside this risk, the main risk here is the volatility of bitcoin, I do not think it would be logical to compare this risk with things like getting your assets affected by a natural disaster or getting stolen in case of gold.
It is also important to say here that I am not discouraging investment in Bitcoin, but rather advising against such investments if the investment plan cannot be successfully executed.
When you invest in a business, you have to invest in various types of raw materials, various machineries with this money. Can you withdraw money from there at any time if you want? Then why should you use the money you invest in Bitcoin for your emergency purpose? You should only invest in Bitcoin that amount of money that you will not need for a long time. You should always keep a separate fund for emergencies. And if you are in such a danger where your life is at risk, then if you absolutely need to sell your Bitcoins, you should definitely sell them. If you sell your Bitcoins for a small need, then it will not be a long-term investment. This is how I explain the matter.
Yes, I didn't understand the first line of your statement. What did you actually mean by that?
Even then, from what I understand here, if I answer from there, people use funds invested in Bitcoin to handle emergency situations or to meet their income needs when they invest in Bitcoin beyond their means, meaning when people invest an amount in Bitcoin without having a backup fund that they cannot afford to lose or cannot hold for the long term. The main point here is that "investing as much as you can afford to lose" is not only used in gambling, but also in Bitcoin or crypto investment even then it is not the gambling.

I never like to put all your eggs in one basket. You'll be safer if you divide all your eggs into several baskets.