But is it really a huge group, though? Diablo is a franchise from a different age, when gaming was only for people who were willing to step up to a certain hardcore threshold. That requirement automatically limited the size of the audience. The explosion of more-casual gaming over the last decade has created franchises whose sales make Diablo's look anemic.
If this list (http://en.wikipedia.org/wiki/List_of_best-selling_PC_video_g...) is accurate, for instance, Diablo and Diablo 2 sold about 6.5 million copies together, which ain't hay; but The Sims, Sims 2 and Sims 3 together sold 39 million copies, making it a completely different magnitude of hit. (Not even counting all the expansions, either.) And how many hundreds of millions of copies of Angry Birds have been sold?
In absolute numbers, thats true. Still, Borderlands sold 4 million copies and was considered a huge success. So the cost vs. benefit was right, although it catered to an audience that might be small in absolute numbers.
Count in other grinding games that are on facebook (Farmville, etc.) and you will see that the number of players that are entertained by grinding is actually a substantial part of the playing community as a whole. It doesn't matter whether the game is "hardcore" or not.
Seeing the rather extreme case that these many years after release, there are still that many people playing Borderlands (and Diablo) just for the grind is an interesting thing.
I think it's a lesson about the end-game. What do your players do when they've finished your game, and are willing to play it from the start again? What can you offer them?
If you could get your sims characters to craft items with a probalistic quality curve based on their personality, skills, and some luck, and also have an internet-facing display chest, it'd probably give even more legs to the franchise.
Well, there's Pokémon, which sold "quite a lot" and had lots of collecting / grinding.
See also Animal Crossing, which gives people plenty of collecting and display opportunities. There are fossils, furniture, fruit, paintings, songs, haircuts, fish, bugs, etc etc. AC appeals to casual gamers because you can do 10 minutes a day and get enjoyment from it. It also appeals to hardcore collectors because there's so much to collect, and some of it is "rare".
If this list (http://en.wikipedia.org/wiki/List_of_best-selling_PC_video_g...) is accurate, for instance, Diablo and Diablo 2 sold about 6.5 million copies together, which ain't hay; but The Sims, Sims 2 and Sims 3 together sold 39 million copies, making it a completely different magnitude of hit. (Not even counting all the expansions, either.) And how many hundreds of millions of copies of Angry Birds have been sold?