It took me a while to realize this, as I began my startup journey over valuing software skills and under valuing business skills, but you truly need marketing, sales, and legal (business ops) in addition to product and engineering. As a solo tech guy you can do product and engineering, but the other areas are neglected unless you can pivot away from development and focus on those.

Small, highly profitable companies are totally feasible. But making a big splash with a product (not just consulting / freelancing) is extremely difficult as a single person.