I work as Software Team Lead in a "seasoned startup" (6 years old, around 60 employees) in Europe. I've been working there almost since its inception, first as an intern, then full time. Two years ago the company pivoted completely and switched from developing product Alpha for market A, to developing product Beta for market B. The reason for the switch was basically that product Alpha was too expensive and nobody was buying it. I've always been skeptical about product Beta, but recently I got definitely convinced that the business idea behind product Beta is fundamentally flawed. This is the main reason why, after six years at the company, I've decided to move on and accept another offer. I simply don't believe anymore that this startup is going to be successful.
I did not communicate my concerns to my manager before accepting the new offer, because I felt there was nothing he could have said to make me stay. I regret this choice, because my manager (with whom I have a very good relationship) was completely taken aback by my resignation. As a courtesy to my employer and to preserve good relationship, I've given a longer notice period than what I'm required to (four months).
When handing my resignation, I was very vague on the reasons for it ("I decided it's time for me to move on..."). I know that I don't owe him an explanation, however for the sake of maintaining a good relationship I now wonder if I should just be honest and tell him I don't believe in the company.
So to conclude my question is:
Should I tell my manager that I'm leaving because I don't believe in the company anymore, or is that just going to hurt future relationships?