Let’s say your family is poor and could use extra help. It seems selfish to deliberately take a lower paying job when a higher paying job could help your loved ones.
Also what happens when you meet someone and want to start a family? Although the author may be ok with certain constraints in living, their spouse and children may have different preferences. In that situation it also seems selfish to pursue this low income life.
All in all the essay seems to work well for people who are okay with living alone and apart from their family and loved ones.