Why is classical waterfall model not followed in practice?

Answer 1

The main reason why the Waterfall model isn't typically used in practice is because it's unrealistic for modern software development. It works best under the assumption that requirements do not change for months to years, which is generally untrue.

The Waterfall methodology generally proceeds from an initial planning phase, then to analysis, design, and finally implementation. In this case, the software should be guided from planning to implementation with no backtracking, and once the software is implemented, then it's done. This means that allof the systems requirements must be firmly established at the very beginning down to the smallest details. This is generally very difficult (if not impossible) to do, as even the clients are bound to miss some details of the application however small they may be. The software would then go through a lengthy programming phase, followed by implementation. These two tasks combined could take years to accomplish depending on the size of the software/system.

In the mean time, the environment is constantly changing meaning that the software's requirements are also changing. This means that by the time the software is implemented, it will probably be outdated. Not to mention, there may also be requirements that were missed earlier on. These flaws will need expensive post-implementation programming to fix.

So overall, the Waterfall model is a clumsy, expensive software development methodology, which is why it's generally not followed in practice.