What sense it made to do something like Instagram? There were already N social networks where you could share photos. No technical excellence was needed. It was just momentum, being in the right incubator, and so forth... I understand what you are saying, but it has been always like that.
Well - no. There are some products where the product itself was relatively simple to build, and the rest was product-market fit. Those are the easy ones technically, but that's not the only type of successful product. YouTube wouldn't be working today if it broke all the time under load.