Most crypto debit card either allow, either enforce the top up to be done - at the end - in actual fiat currencies - usually USD/EUR/GBP, maybe others too in the Asian countries.
Some allow you send there only Bitcoin or certain altcoin, some allow you even keep stable coins too on it - Binance card allows you have both BUSD and USDT.
So... unless I've missed something - and then you may need to explain better what you actually want - the feature you are asking for is, in a way or another, already present in some of the crypto cards.
We dont need actual fiat currency eur/usd...we should be start using stablecoins like fiat currency.
Only what we need from the old fiat currency is CASH but we dont need even banks we need wallets and debit cards.
Here you're also wrong. Shops need fiat. They have the accountancy in fiat, they sell for fiat. And this means that the cards will have to send fiat to the shops for the things you're buying. Some allow you keep crypto and convert to fiat at the moment of spending, some will ask you have already converted enough funds you want to spend. It's up to you to choose the card that suits you best. But, whether we like it or not, fiat is still there.