— You say, that there two options: to hire a good Senior-level specialist to build a prototype or to assemble a team of the Junior-level developers more or less suitable for the role. In that case, why it's better to build a prototype when you can hire an experienced developer and build an MVP?
— The prototype is an essential part of the launch of any more or less well-made product. With the help of a prototype, you check the business-model and understand whether it's worth investing in the product or not, without spending a lot of money and time in advance. Therefore, you shouldn't do MVP right away. A Senior-level specialist will cope with any task, but his services are expensive, and, as I said, it's difficult to attract a specialist of this level to a startup. Why spend so much effort when you can hire a novice specialist, whose knowledge is quite enough to build a prototype.