Ajay: Practically speaking, digital marketing is relevant for majority of businesses to fulfill various marketing objectives such as expanding reach, brand building, thought leadership, lead generation, customer acquisition and reputation management.
Technically speaking, for businesses whose target audience is not yet available on digital marketing, digital marketing is of no use. However, when I say 'target audience', it includes all of the following:
For e.g., in case of McDonald's:
User is entire family
Buyer is parent
Influencer is kid
Normally, people only consider user when they think of target audience. If you don't apply above distinction, you may ignore digital media for scenarios such as promoting diabetic drug/treatment for old age people. They themselves may not be present on digital media such as Facebook but the buyers & influencers, which are typically their kids are surely on digital media and thus digital marketing is relevant.
Hope this helps?