Here are some more things you should be aware of:
1) the airline doesn't check if I have a visa and I'm able to board the flight
Greg and Zach both mentioned correctly that airlines do check your eligibility to be landed at the destination; as they are fined and are responsible for repatriation if you are inadmissible.
2) the visa will be approved
This a dangerous assumption to make, and as per #1, a moot point.
3) I am flying with only carry-on bags
This really doesn't have an impact on your waiting in the transit area. At some airports, you can freely move between the transit and the landing areas. In others, transit areas are limited, restricted, or may be closed entirely.
4) I already have an international SIM in my phone
Not really relevant to your original question, not sure why this is even a concern.
5) if the visa isn't approved for any reason, I could just buy an onward flight to somewhere else.
You are assuming that you will find a sales desk in the transit area for the airline you want to take. This is risky. In my experience, transfer desks in transit zones don't sell new tickets (they can only assist with existing reservations and upgrades / downgrades). I have had many people whose flights were cancelled try to buy tickets on another airline but the agents were unable.
If an onward ticket is required for your visa, this will also be checked by the airline before you are boarded.
Gone are the days that you could buy a ticket from the gate agent - these days you need a ticket just to get past security.
Would it be helpful to buy a cheap onward ticket for the next day, just in case I get asked any questions about why I'm waiting around in the airport?
At some airports you may be questioned if you look like you don't belong - as to what you are doing in the transit area. A ticket may not help you, since you would need a boarding pass to really justify your presence in the secure area.
In some airports, there is a limit on how long you can stay in the transit area (before you must be landed or depart).