One of the most important traits in a successful startup is adaptability. Over time, you’ll face new competitors, and your existing competitors will launch new products and services to try and out-compete you. New technologies will emerge that could drastically change how your business operates (or even render your business obsolete). Market conditions will change, and consumer preferences will change with them. And as a result of this confluence of factors, even the best research business plan will quickly become irrelevant.
If you’re lucky, these changes won’t have much of an effect on your core business model. You can continue operations as you have in the past and still make money. But more commonly, you’ll have two real options. You can either adapt or watch your business fail.