Fact: Exactly 1024 ports are inforwardable, 1-1023, that makes 64511 ports free to be forwarded by anyone.
People can forward up to 5 ports.
This applies only if you assume every user forwards five ports. I'd say the vast majority of people use 0-2 ports and a vast minority is above the 5 ports mark, simply because there are so little use cases for more than five ports. Most people want to torrent, that requires one port. All this renders the extrapolation somewhat difficult and needs some mathematical function to describe that is by no means linear, unlike the one you're trying to plot.
That would make the feature unnecessarily difficult to manage. Besides, OpenVPN does not come with a feature like this, and neither does Wireguard. So such a feature would be limited to AirVPN clients, robbing everyone of their freedom to choose the client software.
Warning: Two or more sessions are connected to the same server. The same IP+Port can't be mapped to multiple destination.
Client area. It is a limitation, yes, but I think it is impossible to deny an OpenVPN client the connection if such a situation is encountered. OpenVPN does not know AirVPN's infrastructure. So if you see that two connections of yours are pointing towards the same server, reconnect with one of them and port forwarding should work again.
It is never lost, it's always there. It just isn't possible to provide that feature in situations like above.
I'd say, check your own setup first before pointing fingers at others. Likely there's something happening with your OS.