Is this allowed?
A business sole based airline - you have seen a few time and again pop up over the atlantic. Or doesnt it work?
What’s exactly your question? If you want to know if an airplane is allowed to fly only with business seats - the answer is yes. Same for first or coach class.
It can be hard to make work, for similar reasons to in real life - you need to fill an entire aircraft worth of Business seats; your competitors will often only need to fill 20-30% of their floorspace, and can cross-subsidise from their economy cabin when competition gets tough. You’ll likely have fewer online connection options, because only do many routes can support a full load of Business passengers.
The way that ORS works at present makes it a bit harder, because it more or less shares passengers between flights with the same score and caps at a maximum of 100 - once competition heats up, it becomes very hard to fill an entire cabin.
if you're going to try such an adventure make sure you start it at low AGEX; a decrease in AGEX hits premium cabin demand the hardest. So you might find that all sorts of route work well during high AGEX but once the cycle comes back down you are out of luck
At first I think CS1/CS3 made all biz-class airline possible on AirlineSim, but in the end it is still too big and difficult to fill!