Best Gearing for a very fit but on his way out 52 year old Firstly, I'm no beginner.
I've just bought a so called good bike from a major retailer.  Why would a "good" bike come with "beginner" gears?  Chainring 50-34 with 10 speed cassette 12-28.  I enjoy terrain that is similar to turbo training.  That is, get into a rhythm and fight to maintain it for as long as possible, changing gear one tooth to maintain cadence on slight up hills or into wind.  I will never opt for up and down big hills.  Not my idea of fun!  I enjoy hitting high speed and maintaining that....  But at 52 I'm not getting any stronger with time.  I'm thinking of switching to 12-25 just because it has more gears in the middle range.  but I now probably need a 48 on the front so that the ratio 3 +/- 0.3 is in the exact middle.  Can I get a 12-21?  What's the point of the smaller chainring?  All gears are duplicated except those at end of range which I will never use?  Why not remove and decrease weight?
I use Sheldon Brown's Gear Calculator whenever I'm looking into gear ratios. For someone who doesn't ride any big hills I would think that 50/34 - 12/28 would be absolutely ideal. You could go 1x and get 98% of a 50/34 with a 44+10/42. See SRAMs pocket guide 

50-12 tops out at about 33mph definitely not something that mortals can sustain for very long on a flat road.