Then you're still in the same boat. Over the timescales that startups typically operate under (weeks to months), one senior engineer is more productive than one senior engineer and five junior engineers. The former can get a lot of code done and produce something that works. The latter spends all their time mentoring the junior engineers, who might or might not produce something that works, but it will undoubtedly be ugly and require much refactoring if it does.

Over longer timescales (years), this relationship doesn't hold true. This is why Google, Microsoft, and many of the other big players in the industry can afford to hire junior engineers. That one senior engineer and five junior engineers will turn into six senior engineers, all of whom have worked closely together, and then you have a gelled team that can accomplish great things.

But you have to get there first. When a startup's fighting for its existence daily, it's not cost effective or prudent to make an investment that will take a couple years to pay off, when there's a good chance that your company will be out of business in six months.