I feel like a life void of luxury is... not preferable.
Penny pinching and then dying a millionaire sounds awful, because what's even the point of having all those millions if you lived like you had nothing the whole time? Just a number on some imaginary scorecard?
Wasting away all your earnings and living a life of "luxury" with nothing in your savings is at least just as bad, because if something catastrophic happens, you got nothing.
Gotta strike that happy balance, where you get to enjoy the money, yet still have a substantial chunk of it saved.
Building generational wealth for your progeny.
GP said their grandfather died a multimillionaire. That money can go to his children; managed well over their lifetime it will double several times over. And learned frugality on their part means they have an even greater sum to hand to any children they have (like GP!)
And if a catastrophe struck - be it a local one like joblessness or divorce, or a global financial crisis, whatever - grandfather knows that his austerity guaranteed his children shelter, food, and education in the future. That's a powerful reason to make decisions. Hell, in our animal brains, that's probably one of the most powerful drivers of decisions: what should I do to benefit my family?
The point is, is it worth it if you do not get to enjoy even a part of that at all? What if you don't have children? Assuming there is nothing after death, the personal result of being a penny pinching millionaire vs. just not having that much money at all would be the same.
Presumably he had a good family/friends, which is what matters, no?
Investing in your family's future is an incredibly noble/selfless thing to do, it takes real character.