"What is the alternative to this? If not work - what is important? what makes one person a success and another a failure, and what gives purpose and dignity to life, in your opinion?"
First, there is the definition of success. For any given species or individual of that species, simple survival is the first level of success. This includes basic shelter, food and water sources, and protection from dangers.
Beyond that, once our more immediate needs are met, I think the next level of success is breeding. At one point, during my "Descartesian reset", I determined that throughout most of history, the primary purpose people had was to reproduce as a way to ensure their own well being in old age, but also as a sort of abstract, indirect immortality through their offspring. There is a reasons there are many cultures who still largely center around first-born.
For many of the proletariat, the safety and security of offspring was ensured through numbers (high mortality rates in the past) and through work servitude that was seen not just as an exchange of labor and time for money, but as an exchange of labor and time as an investment in their offspring, with money only being a tool to that end.
I think that, with the advancement of the arts and technology though, this otherwise ancient traditional model has been largely upended. Kings built castles, and Pharaoh's built pyramids, and other builders build structures that would last well past their deaths as their version of immortality, but normal people always were mostly just reproducing their dna. I think this is also why there is still so much reverence for glory and honor, particularly in battle, in that, if dying in a sufficiently glorious way, you are more likely to be remembered longer.
Past that, at some point I sat down and made a list. Essentially it boiled down to things that have a lasting impact for the greatest number of people beyond my death. Things like art, architecture, scientific discovery, writing, actions taken, etc. I don't remember the entire list, but it was fairly expansive.
The thing to keep in mind about all of these is the indirectness of their totality, that it to say a piece of art in itself may not actually be that big of a deal in the long term, but lets say that piece of art inspired another persons mind to do something great that they might not have had the inspiration to do otherwise.
tldr; Essentially, I think it all boils down to humanity's ancient tradition of trying to cheat death, and the way we do that has been changing. To me, as an atheist, the afterlife isn't a place where I exist, per se, but is more like the butterfly effect of my consciousness upon the universe it experienced before it's having been extinguished.
Of course, I'm hoping I can get my hands on a telomere regeneration mod to buy enough time to transplant my consciousness into a hopefully by then sufficient computer and kick off the singularity, but I digress.
To answer your original question, it isn't the work in itself that gives purpose. It's that a purpose is found and then striven for through whichever means are available, which for most people is usually indentured wage slavery. Ergo, when an older generation sees not working as a lack of purpose, do not mistake it that they think the work gives purpose, but that the work supports an already existing purpose. (which for most of them was child-rearing)