Since you used business as a reference, I believe those business cannot hold each bitcoin sent or rather they'll sell cause demand never seems to end and just imagine you keep holding bitcoin what will you use to buy the goods, note you're dealing with business not personal use, I believe each time they get paid with bitcoin they convert the bitcoin to fiat to be able to rebuy those goods.
Hardly I see small shops accepting bitcoin and mostly supermarket that accept bitcoin as a means of payment so you don't have to worry seeing a mall or supermarket accepting bitcoin, in any situation where you see a little shop accepting bitcoin then the person is a bitcoin enthusiast and they also have some strategy of handling such.

There are only 2 ways to get around this.

1. Exchange crypto to fiat on regular basis so that fiat value doesn't change much.
2. Keep holding until you are in profit and then exchange to fiat.

Depending on the individual's preference they can opt for any.

Yeah and that's the reason why other businessman and businesswoman are too successful cause they are accepting bitcoins for the mood of payment which is pretty good enough cause they can hold the Bitcoin until the price will go up higher and then the merchant get another income for the prices going up. But for me it's hard to do it cause we all know that Bitcoin is for long term so there's a chance that you can not hold all your Bitcoins from the buyers cause you need to roll out your capital in order to grow your business.

If they will convert the Bitcoin to fiat or stable coin like USDT then why are they accepting the BTC payment method? and why not just accept fiat if they will convert it to fiat immediately?

We cannot completely say that this is why most companies accept BTC payments, but we can predict based on our points of view.
Bitcoin price is volatile and everybody knows, these companies accepting BTC payments are probably into bitcoin investment so that they will use that method to accumulate their desire for Bitcoin instead of going through p2p or going to centralized exchanges to expose their privacy. They will have extra money for their business to replace all the payments that are been made through the Bitcoin payment method to hold the BTC they accumulated.

Small-scale businesses without big capital may not survive this BTC payment method especially due to network congestion, market volatility, and price instability.If such businesses accept the BTC payment method, they will end up selling the bitcoin immediately to keep the business moving, and doing so might affect their business because the Bitcoin price might drop down before they sell.

I believe any company or organisation who decides to accept Bitcoin as a means of payment already knows what is at stake and how to go about it. If Bitcoin payments was giving them issues, they would have scrapped the idea out immediately. It will be very difficult for small businesses to succeed with Bitcoin as a means of payment, considering the price volatility of Bitcoin, except the business owner just wants to gather Bitcoins temporary for holding while running his day to day business with Fiat.

For big organisations,  they can actually do this without feeling the impact. It's either they hold Bitcoins in their reserves or convert to Fiat immediately since the value the customers will be paying in Bitcoin will be same in Fiat.