You mean Aldwych rather than Aldgate, but, to answer your question, no there would not be a huge demand for such a service during the day. In day time hours, the N9 route would be much slower and probably take in the regions of 1.5 to 2 hours compared to about 1 hour at night. The tube is a much more efficient way to travel for a relatively modest overhead if you have an oyster card. The bus would primarily be used by pax traveling shorter distances rather than the full LHR-Central London stretch, and there are plenty of local buses over shorter distances. I would be surprised if there were more than a handful of individuals on any given day willing to spend two hours in this bus to save just a couple of pounds.