I think 90% of people could do without a front derailleur right now.
Really? Even with my 53/39 front and 26/11 cassette I frequently do not have enough range on either end during the same ride. Especially this time of year, doing base and trying to stay in a steady zone 2 my cadence will drop to the 50s in my 39/26 while climbing a big hill when I'd prefer it near 80. Then bombing down in my 53/11 I'm spinning over 110. I know not everyone lives in hilly terrain but lots of people go out of their way to ride hills. Even riding in Florida where it was dead flat I had the pleasure of riding 30+ miles into a 30 mph head wind and then made the return with the tail wind. So even used the extreme ends of my 20 speed gearing there. Related to Ghisallo's post, I find my heart rate to have some correlation with cadence even while maintaining a steady power, so I watch my cadence during base.
On the road I guess I'm in the 10%............and my guess so will be about 3/4 of serious roadies such as ProudDaddy (most of whom appreciate a closely spaced rear cassette / who challenge themselves via varieties of terrain and distance) will also be in that 10%
Yes it could make sense for cross which has a limited speed profile.....but gosh don't suggest on the road unless you commute / tool around on only on flatish roads.