A good instructor should not send you on a first solo without taking into account all factors, traffic density in the circuit being one of them.
Yes, all sorts of things might happen but at the end of the day you have to ask yourself whether or not you feel the student is capable of the task.
Finally, perhaps we should also remember that we seem to live in a world where litigation and liability have become more and more significant. If you are sending a student solo and you have not taken reasonable steps to ensure that he can deal with an inflight emergency then you may be opening yourself up to legal action thereafter should there be an incident/accident.
My father who was a veteran light aircraft instructor also mentioned the "self- survival" factor, ie if you are on your own in an aircraft you will do whatever it takes to look after your own neck!