No, in my logic there are two distinct things: the public paying for concerts (and the public not caring) vs the public caring and supporting them itself.
In the first case, it's just the banks buying some prestige by playing to supposed upper middle class preferences, and get some free publicity plus some tax exceptions for those sponsorships.
>The dimension you are missing is spatial, in that this would now happen elsewhere from London, and the "externality" of non-banker Londerners being able to experience this, is the negative effect referred to by the OP.
So? If the Londoners didn't care for those events enough to support them themselves in the first place, why would they have a problem now that foreigners, and not themselves, will get to "experience" those?