Just because you are a founder, does not mean you "are" the company. To build culture as a company, you need people to "grow up" within said company. To be sustainable for a long term, it's more important to have a team that is familiar with your company, the way you do things and the issues you encounter along the way. Someone "growing up" in your company will be way more valuable long term. Sure, you might say a senior dev will be able to grow much the same way but 1. He will cost more upfront 2. He might have (justifiably) have higher standards of his own and leave you for a better offer 3. He will be less flexible in trying out new things

Those are generalizations but in my experience, once the 5-Year mark passes, a junior is pretty much always a better colleague than a senior.

This is not to say that you should not hire seniors but you need a good mix of trainers and trainees. Training employees can also make a senior much more effective at his own job.