Well, you’re wrong. Level 60 is incredibly fast to reach with how exponential experience scaling works. I can tell you’re not that knowledgeable about what’s going on with D4.
So those 30-40 hours are great, and (to me) very much worth the $70. But the expectation from a Diablo game is to be evergreen. I kept coming back to Diablo 2 for 4 years, then Diablo 3 for 10 years.
This type of games, today, are built with microtransactions so players who keep playing for a long time have stuff they may want to purchase and increase revenue. Replay and longevity should be the core pillars, and the developers had already applied (and invented) the patterns for making such games.
Diablo 4's systems and design does not encourage that sort of attachment and replay. I played the campaign to 71, then the season to 64, and I'm off until the game changes substantially. Diablo 2 & 3 changed radically years after they launched so I'm hopeful, but it's surprising that D4 seems to have ignored the lessons from the evolution of 2 & 3.
Something oddly similar can be seen with the Borderlands series leading into the latest entry, Wonderlands. Amazing campaign, but the replayability and longevity of previous titles is gone.
Stark reminder that developing creative works is hard even if you are a master of it.