Should models be always followed?

Absolutely NOT. Models are attempts to capture and make sense of very complex events and influences. They will not automatically lead to correct answers in all situations, but they help give a framework and to provide suggestions. However, the suggestions can often be refuted. For example, the product life cycle suggests introduction, growth, maturity and decline. However, I don't think you need to worry imminently about the decline of all brands eg Coca Cola has been popular for decades and will probably continue to be. Similarly, in PESTEL, technological breakthroughs, like digital recording, might suggest some companies will not survive, but music recorded on vinyl has a strong and growing niche market.