Well I maybe will have an opposite view in this thread, but I think digital marketing is only one part of the puzzle. I think it's important to learn the fundamentals also, for example do a proper market research is not something I was able to learn alone on internet properly so I guess it's good to know what you are doing and see if their is a market about what you are selling, I doubt learning only SEO is enough, but maybe I am wrong completely it depends of your goals.
I value the local market and would like to sell nationally so it would be interesting to see if my diploma will have his importance in creating my own business... Normally it's the case

But I agree that you should not put yourself in debt and sell one arm only to have a piece of paper, it's the knowledge that is important.