This is a purely mathematical response. Assume 10 speed. Assume 700C wheels. Just for fun. I suppose you could go 11 speed too. And 26 inch wheels too. But 10 speed and 700C. 11-36 cassette. Putting the gears into a gear chart with a 34 tooth single chainring, I get about 22 mph top speed with 34x11 gearing and 90 rpm, 83 gear inches. And I get about 7 mph with low gear of 34x36 and 90 rpm, 25 gear inches. All the gearing jumps are about 5-6-7 gear inches apart. Not bad. Only problem is the high is not too high, not really much of a problem. And the other problem is the low is not too low really. 25 gear inches isn't anything to brag about for a low gear. You could run a 32 ring and get a high of 78 and a low of 24 gear inches, 21 and 6.4 mph. But the low really isn't a lot better and the high is definitely worse.
The problem with running a single chainring is the gearing is compromised. Either you lose high gears, or lose low gears, or lose the nice progression between gears. You cannot have all three with a single ring and 10 or 11 or even 12 cogs on back. SRAM is making a 12 cog cassette now. The SRAM 12 speed cassette is 10-50. The SRAM 11 speed cassette is 10-42. I suppose with a 32 ring and 10-42 cassette, you could get a range of 86 to 20.6 gear inches. 23 to 5.5 mph at 90 rpm. Good low gearing. But not a great choice of middle or high gearing and big jumps between gears. You got to compromise with a single chainring. With a triple crankset, or even a double crank, you can have nice high gears, nice low gears, AND nice jumps between gears. You can have it all with a double or triple crank, you can't with a single ring. You give up good gearing range or progression for the benefit of maintenance, reliability, ease of shifting.
