If I'm riding even a 40 tooth front cog, I find that I never use the smaller cogs on the rear cassette. My plan is to eventually use a 32 to 36 tooth front ring only with no front derailleur at all. This will make my bike an 8-speed. With that setup, the bulk of my miles will be done with the chain in the middle of the rear cassette, with some use of the smaller cogs in the rear. This will improve my chainline, and will still leave plenty of "low-low" gear available for the rare "climb" that comes along. There just aint no mountains in Louisiana!
you say you ride on only one chainring, and you show the bike with the chain on the small ring, so presumably that's the one. In that case, wasn't going to a compact crank a move in the wrong direction? If you need more gear for "the flatland," you'd want a BIGGER chainring, not a smaller one.