The production of PCB prototypes is a process limited in time: it should take as little time as possible to make way for real manufacturing. Companies that offer PCB prototyping services are also probably involved in manufacturing. As such, these companies strive to develop good / perfect prototypes.
The creation of PCB prototypes is an essential process that precedes mass production and the subsequent launch of new PCBs in the market. This process checks whether a suggested PCB design will work as expected, revealing possible flaws and allowing designers to make the necessary changes. Any other improvement or alternative that designers can think of is also presented at this stage.
The mass production of any new product that neglects PCBs can not simply start without the prototyping stage. The creation of PCB prototypes, in this case, will be used to confirm and reaffirm that PCB performance is consistent. The manufacturer or customers can give their opinion about the PCB along with any other suggestions / improvements they may have.
PCB manufacturing companies should always be prepared for any possible outcome and, therefore, any modification at this stage should not be considered as an additional cost. Ignoring the creation of prototypes and getting involved in the mass production of a PCB just to fail will certainly be catastrophic in terms of effort, time and money wasted.