chasm54

I don't know about insights, but as usual I have plenty of opinions.

First of all, you say this road was close to flat. So the downhill gradient was only very shallow. In which case I am not at all surprised that you found the 53x14 or 15 the most suitable gears. I wouldn't expect to be in my highest gears in those circumstances unless I was flat-out sprinting.

But I don't think that justifies the conclusion that you don't need the 53x12. Or, to be more precise, you may not need it but that doesn't mean it isn't worth having. You'll go down steeper slopes than this, and on those it may be useful. In particular, you mentioned an interest in time-trialling. Were you to pursue that interest, you'd definitely want a gear that would allow you to continue putting a lot of power down when going down a moderate hill.

And really, what is the downside of having the 53x12? Unless you are having trouble with the hills in your 39x25, why would you change? And even if you changed, you could go to a 12-27 or a 12-28. From your account, it isn't clear what problem needs solving, here.