> Given that so many developers start with JS these days, this becomes standard practice and they don't even think about it.
Bit of a logic jump there. The model that you accurately describe, while far from perfect, has the advantage of significantly reducing cost and complexity, speeds up prototyping, and ties in well with continuous delivery approaches which from an economic point of view are far more efficient than delivery management or waterfall models.
Yes, there are also disadvantages which mean this approach is not suitable in every case, one has to know and assess the risks.
In particular, if your organisation is middle-management heavy this is not going to work. Then again, if you're in such an organisation you're pretty well fucked anyway.