I see your point and agree with your rule of thumb, but don't think that BA will be fine just filling their not exactly inexpensive to operate RJ100s on two daily flights (morning in, evening out). The assumption would be that indeed those two flights are a gold-mine. On routes with competition and excess capacity (ZRH, GVA, MIL, FRA, EDI etc. etc.) I tend to believe that it is difficult for BA to make the rule of thumb work. I regularly check the fares on the FRA-LCY route, for example, and BA's fares are quite reasonable, indicating that BA is not able to syphon off just the cream.
I guess it pretty much depends on individual routes. VLM and EAE pulled the DUS/MGL route because they could only operate two daily flights (morning in, evening out) with decent loads. The same was true for most of the EW routes to Germany. VLM tried to trigger demand by selling cheaper tickets on their late morning and early afternoon flight without lasting success (despite operating LH code-shares, offering Miles&More and having a monopoly on that route). Then they cut back to thrice daily (like LPL now....) and struggled on for a while before eventually scrapping the route because they simply had not enough aircraft utilisation (so they told me). The problem was that they had no chance to make use of the aircraft when it did not make money to/from LCY because they had no network at the other end.