I want to believe you, but it seems like that line (Act "where you are now" is easy, just wait until "Difficulty level higher than where you are at" + "Later Act") is getting tossed around a lot. When really the bottom line is that players are adapting to the difficulty more quickly than the devs intended and that defining line keeps getting pushed back. I think there are a few issues that either the devs didn't forsee or gave up on addressing.
1) Players these days are a lot more knowledgeable about game systems in general.
Even if you make an uber difficulty level, people are pretty good about finding dominant strategies and putting them into use. Plus, once one person finds them they're good about sharing that information which leads me to...
2) Players are a lot more connected these days.
Once the dominant strategy gets out, it takes only a few clicks to find that information for yourself and then adapt it to your game. I mean ten or fifteen years ago, half the fun was trying new builds and experimenting for yourself. Now you can just go online and reap the benefits of someone else's efforts. Also leads a lot people to worry about what is the "best" build or are they "doing it wrong" by choosing one skill or another.
3) The auction house will make things too easy.
In a game where finding / grinding loot is the carrot on the stick, giving people the option to sell / buy loot that other people have found is going spell an early death for this game. I don't even think they'll get the chance to really profit off the paid auction house because once players get to a point where their characters can farm Inferno, the market will dry up (because there really is no need to level up more than 1 of each class.)
I don't know, the whole thing just smacks of them going for the quick buck. I can't believe that they'd be this blind to the ramifications of the skill progression system, influence of auction house, and lack of meaningful stats - so I'm left wondering if they're just inept or gunning for the short term profit.
Hope I'm wrong! I was planning on playing this for a loooong time. May have to fall back on hardcore and self-enforced rules though.
Too be honest, I'm a hardcore sorc player from D2, but I'm just DYING to take the Witch Doctor into hardcore. I don't know what I'm going to make yet but the class just looks like fun with its zombie clowns and curses.
For starters, Diablo II had no additional revenue stream outside of the box sale. Although it did support LAN, most play was through Battle.net. So they were basically incurring an expense with no revenue stream to pay for it outside of expansion packs. The longer the game went on, the more it actually cost them.
Because they now have additional revenue in the form of RMAH, it's quite possible they are perfectly ok with the game not lasting 10 years. As long as it's profitable and runs its course, whatever course that may be, the game can be considered a financial success. Sure, the longer the better, but that doesn't mean, for example, that 5 years would be considered a failure.
Don't forget, of course, that patches, updates, and expansion packs are inevitably coming on a regular basis and will no doubt add a lot of things to the game. Even D2 is nothing like it was at launch. Every single thing people currently complain about could be a distant memory after just a few patches. Maybe not, but saying stuff like "I can't believe that they'd be this blind to the ramifications of the skill progression system" is just one of things where I want to facepalm, because it's not like they have a bunch of invalids working there who are reading forums and going "OH MY GOD I CAN'T BELIEVE NOBODY EVER MENTIONED THIS. WHAT HAVE WE DONE?".
It seems like this is a recurring pattern with every single new game that's come out in the past 5 years, where everyone gets all crazy when the game first comes out and acts like the developer never even thought of the most obvious issues in the world, and then within 6 months everything is fine. The only counterexample where the game was actually completely doomed to disaster and failure was maybe FFXIV.
Anyway, bottom line, everyone should chill and wait for some patches, some developer interviews, E3 maybe.
As it stands, there is plenty of content to keep everyone busy for a while, even if during that time you have concerns about the longevity of the game.
BTW, you mentioned there is no reason to level up the same character class more than once. Technically, there is 1 achievement for each class that you get when you level the class to 60 twice.
coming from the perspective of a developer, one thing I can tell you is that a lot of time the things that feel "off" on a game's launch were simply a matter of schedule. Even things people consider simple, like the messed up itemization right now, require a lot of time and balance to get right. But things like this can easily be patched in later, maybe even in the first patch, and it goes away. It's not a fundamental design flaw, it's a simple balance flaw.