Go to the interview. Other than travel costs, you have nothing to lose.
You're content with what you have. Whether you get an offer or not, you have a degree and a career already so you have nothing to worry about.
Get the offer. Then and only then can you make a decision on what path you want to take!
In terms of potential lost earnings, that's exactly what they are, potential. You could argue that a career in medicine would increase your potential earnings towards further in your career.
But I can fully understand not wanting to dive into another 5 years of uni when you already have a career which you could be progressing in. It all depends on how much you want to do medicine and how happy you'd be to continue doing pharmacy. This isn't something anyone on the forum can help you with as it's your decision based on your feelings.
However, we can help you with the decision to attend the interview and that's to go to the interview!
If you don't get the offer, nothing has changed. If you get the offer and decline it, then you've turned away from medicine on your terms. You've declined medicine, not the other way. How's that for closure! And if you get the offer and study, then you're working towards the degree and career that you so wanted from 18.
That's my take on it anyway. Either way, good luck!