Road Triples (7/8/9spd) were limited to a 30t small ring in most cases. Most of the compatible rear derailleurs could only handle a 27t rear (though, in most cases, everybody rode 25's). A 27t got you down to ~7mph at 80rpm. Pretty dang low. Low enough for touring bikes for a couple of decades.
However, extra chainrings can make a mess of front shifting, often causing more frequent chain suck and more frequently dropped chains. This is why triple cranks are beginning to disappear.
|This was a common sight on triple-equipped bicycles in the 90's and early 2000's.|
Chain suck is something that has plagued triple drivetrains from day one. Partly because of the stresses and tight chain angles of a gear shift between two sprockets of very different sizes, and partly of the excessive wear on the middle chain due to its use for almost 90% of the bike's mileage. On a typical double the chainrings get used more evenly by the rider, increasing the life of expensive chainrings and helping prevent incidents like the one pictured above.
This is why double cranksets are awesome: They are simplifying the bicycle's drivetrain and your climbing gears are still getting easier. It turns out that bicycle engineers know what they're doing.
Newer compact doubles (34x50t rings) can offer even lower gears than the triple setups we were used to in the 90's. Sram's long-cage road derailleurs can shift into a 32-tooth cog at the rear. If that's not quite low enough, Sram 10spd mountain derailleurs are compatible with their road shifters. That combination allows you a 34x36 low gear, which gets any bike rider down to a walking pace. This is why Sram does not offer a road triple: Nobody rides slow enough to actually use it.
As for mountain bikes - yeah, we needed 3 chainrings back when our lowest gear was a 22x32. Actually, I remember struggling to stay upright when I actually climbed in that gear - it was only marginally faster than walking. Now we have mountain doubles with even easier climbing gears - as low as 24x36. That's 17 gear inches. I can ride slower than I walk and I never seem to drop or jam the chain the way we all used to with triples.
|Modern 2x drivetrains make this climb doable.|
Funny thing, though - no matter what I do to my bike, the top of the climb is always higher than the bottom.