After my gf died, my gm never showed any interest in another man. I think she enjoyed life without a man in her life, since all she knew was servitude. But you're right, every R has it's own formula for success. I suspect my gf would have said his M was great, because his W served him as he expected. My gm might not have agreed.
I think it's telling that this is how you feel about your grand parents relationship.
I understand you believe that your gm served your gf throughout their M. And I understand you believe your gm never was with another man after your gf's death because she did not want to be back in a serving role.
And, I wonder if your gm believes the same. If she's stated it, or if you are just assuming it. And even if she's stated it, does she really believe it or is she simply rationalizing something that she chose to do... for what ever reason she chose to do it...