Question about phd interview gone wrong
I think you clearly sensed the concern that the interviewers might have with a candidate who is capable but seemingly unfocused. PhD completion rates are fairly low and one contributing factor is the motivational slump that occurs after a couple of years with very little light at the end of the tunnel. One way (that committees try) to avoid these unfortunate scenarios is to select candidates that are intrinsically motivated by their topic: whose passion for the subject itself will sustain them through the bleak slog that is methodology.

The problem is basically that, as you say, if someone held a gun to your head you could commit, but your advisor is not allowed nor obliged to do that. You need to decide independently whether you are honestly, deeply motivated by the desire to tackle hard problems (and make very little progress) in your subfield.

If it is the case that complex event processing in finance really is the field that will get you through years of low pay and minimal recognition, then you could send a follow up email along the lines of "Thank you for your time. I appreciate your consideration. Upon reflecting on our interview the other day, I feel I may not have appropriately conveyed my interest in SUBFIELD_X, which I think is important for VALID_REASON_Y" However, you should not be terribly surprised if this doesn't dislodge a negative opinion.

Edit: I should of course add that in many, many cases the specific (METHODOLOGY x SUBFIELD) that you are deeply passionate about today, will not ultimately be the one in which you complete a thesis. This is okay, as long as the passion carries over.