I agree with most of what you said but I will disagree with you on the part I bolded. What would you do if you suddenly became unemployed and your investments suddenly lost money due to negative market movements? Then you will see that saving will help you a lot and it is not as bad as you think. No matter how many investments we have or how many sources of income we have, owning a savings account should not be overlooked because life is always full of surprises and unpredictability.
By the way, diversification is a good thing because it helps us minimize risk but is it necessary if we don't have too much money and should instead focus on a single investment to maximize profit? Diversification helps reduce risk but will also reduce our profits, so it depends on each person's financial ability to make appropriate choices.
When they say "don't save your money in the bank", they don't mean you should go and invest all your money in all sort of things. There are moderate investments and other financial institutions that you can buy and have your money back with guarantee like bonds and treasury bills, your money is in save hands and the ROI you get might be big but it covers your asset from inflation that may come later in the future. Saving money in the bank can never protect you from inflation.
Beside you don't have to use all your money to invest in one investment, you can always diversify your money into different investment to reduce the risk. We have things like Bitcoin and some other stock, there are some of them that perform very well and they will help you protect your money but bank can never give you anything, they are using your money for their own financial gains and perhaps even give you some miscellaneous charges such maintenance and debit cards which is annoying.
I'm not saying that keeping money in the bank can protect me from inflation, I'm just saying that savings will help me in emergencies, meeting my daily needs.
As I said, diversification will help you reduce your risk but it will also reduce your returns, so it is up to each person to invest in one or more investments. For example, if you invest entirely in bitcoin and you invest 50% in bitcoin, 50% in gold, which investment will yield higher returns? Reducing risk means reducing our profits, so it depends on each person's preferences.
You need to know that a bank is also a business and they are in business, they have no responsibility to spend billions of dollars to build systems and serve you for free. If you feel dissatisfied with the service they provide, do not use it and there is no need to criticize them. If it were you, would you be willing and generous to create a multi-million dollar system to make it available to everyone for free? Like the bitcoin network, will your transaction be completed if you don't pay the fee? Nothing in this world is free and if you feel they are too expensive, skip them because they don't force you to use their services.