SaaS is not the only business model out there...
It's entirely possible and very realistic. I built a SaaS product in the equivalent of about a 7-8 working days that 4 weeks in had $1000 in MRR and 8 weeks later is double that.
http://joshpigford.com/baremetrics-stripe-analytics
It's about solving problems that businesses are willing to pay for. Doesn't matter how good the idea sounds. Only matters if businesses will fork over real, tangible money for the problem you're solving.
It doesn't seem to me this is very realistic.
My impression is that building any kind of serious SaaS is going to be at least a 6-12 month project for a single developer.
Even ideas like blogs with affiliate marketing revenues probably take about a year to generate enough users to make sufficient money to support oneself and simple pure ad supported sites seem to require at least 100K+ users to make money (ie with RPM's around $1).
Is it really possible to start a software/online business that will earn significant revenue in 2-3 months ?
EDIT: Excluding consulting/freelance work of course.