Back in the days when 5 speed was the technology and getting a double in the front with a range which your derailleur could manage without doing itself a mischief, you did well to have a 27" gear (on 27 X 1 1/4 wheels) combined with something that would let you do a reasonable speed on the flat. By modern standards the jumps between gears were huge, and you did a lot of freewheeling. If you wanted to be able to twiddle up hills, that was what you did, before mountain bikes came along and gearing technology improved. Even so, with a triple on the front, off the shelf tourers still might not have low enough gearing as they came. You might still have to experiment, and one of the conclusions was that the stated capacity of deraillers from manufacturers could be stretched a bit. You could often go from 32 to 34T on the back for example, without issues.
How low is too low? On two wheels I've had a bike with a 16" bottom gear and although it was rarely used, it was usable as long as you actually were able to keep rolling above about 3 mph. It was more useful to fit a larger chainring which raised it to 17 1/2" as a little change at the bottom end gave a gear about 9" higher at the top end. The set up was a SRAM 3X8 hybrid hub gear on a 20" wheel, one of the few situations where I've felt that raising the gearing was beneficial. This was actually a recumbent, but the principle is the same for anything.
When I bought a used Brompton 3 speeder a few years ago it felt like going back to the 1950s as far as the gearing was concerned. First was in the low 40s and I did some experimenting with it before I got it the way I wanted it. The town where I live has some steep ups and downs and if your gearing is capable of getting you across town, it'll be good for most places. I ended up adapting it with a 28T ring bolted to the inside of the 50T ring that it came with as the lightening holes matched the BCD of the small ring, with a 14T at the rear. I made up a spacer to move the BB fractionally to the right. It works fine, doesn't interfere with the fold as long as you remember to put the chain on the big ring. It gives a bottom gear of 24.66" which with the direct drive of the Brompton gets me up pretty much anything. It's a greasy finger change at the moment but I have a suitable adapter clamp so I might get around to bodging up a front derailleur in due course.
