Long, long time ago pretty much all road bicycles had 10 speeds - 2 chainrings and 5-sprocket freewheels. Over time, bicycle drivetrains developed into 2x6, 2x7,... and today's newest 2x11 systems. That's what you would see now on a typical, new road bicycle.
When mountain bikes became more popular, they needed more gearing range and a standard 2x5 or 2x6 system couldn't provide that. The solution was to add an extra chainring so soon 3x6, 3x7 and 3x8 setups became a norm. But when cassettes got more and more sprockets and a wider range (such as 11-36T), it turned out that a 3x9 setup was usually not needed so the extra chainring was abandoned. This is why today's mountain bike feature 2x9 and 2x10 drives and a 3x? (three-by) setup is now a rare sighting.
The situation got more interesting with the arrival of 11 speed cassettes in the mountain biking world. The extra sprocket allowed for even wider range setups and when SRAM came up with a 1x11 system, with a single chainring and a 10-42T cassette, it became obvious that front derailleurs may soon be extinct.
A typical 2x9 MTB setup compared to SRAM's XX1 1x11 - same range but no front derailleur, simpler shifting and a lighter crankset (Simulated using 27.5" wheels with 2.35" tires. Source: gear-calculator.com)
But what's good for mountain bikers, is not necessarily good for roadies. Mountain bikes need wide gearing range and clearly 10-42T cassette can provide that. But it's not very useful on a road bike because jumps between the gears are just too wide. Not yet - but in several years when 12 and then 13 speed cassettes show up, such drivetrain is definitely possible on a road bike, providing wide range and tightly spaced gears.
Today's 2x11 road setup compared to a hypothetical 1x13 - similar range with tightly spaced gears in the higher cassette range. (Simulated using 700C wheels with 23mm tires. Source: gear-calculator.com)
Both 1x11 and 1x12 drivetrains have many benefits - they can be lighter, requiring less maintenance, easier and faster to use. But there are issues as well. The main one is the tight space available on the rear hub. Adding more sprockets to the rear wheel means that the spokes from the drive side must be aligned nearly vertically and asymmetrically loaded. In order to solve this problem, future rear road hubs may simply become wider, for example, adopting the 135mm MTB, or wider, standard. Such transition happened in the past already - old 5-speed road wheels used to have 120mm rear hubs until the newer 126mm and eventually 130mm standards were developed.
My prediction is that in the nearest future we will see more of:
- 12-13 speed cassettes combined with a single chainring
- lack of front derailleurs on most new bicycles
- wider standard for road bicycle rear hubs (and possibly MTB as well)
- more options for wide-range 11 and 12 speed cassettes (plus lower price)
The setup I would like to see for my future road/touring bike - wide range 13-speed 10-40T cassette with a single 38T chainring. (Simulated using 700C wheels with 32mm tires. Source: gear-calculator.com)
The future begins today. SRAM has already showed what the next road setup may look like with their newest 1x11 system. But at this point it's not a particularly interesting offering for those who would want to replace their 2x10 setups - it doesn't have enough range, using 11-speed only cassette.
We need more gears on the rear hub and I'm pretty sure we will have them (At least on road bikes. MTBs may get other solutions such as the Pinion gearbox instead). Anyway, the future looks exciting.