Less to do with number of flights, but more to do with the total number of premium pax per day, as well as the cost/availability of alternatives IMO.

I've done the CPH route about a dozen times over the last couple of years and CE was rarely packed or beyond 4-5 rows.

And BA is making substantial investment in lounges on routes with strong premium demand. An example being the complete refurbishing and "Galler-ising" of the old BOM Terraces into what is now a rather substantial and decent offering. Similar changes have been made at lounges in some US ports, BRU etc. Nothing wrong with focusing on strategically key routes, especially those with strong premium demand. Seems like a sensible strategy indeed.