I hear what you're saying, and I'd love a longer "run" myself sometimes, but I fear due to the very nature of Descent it would be very hard to do.
Descent 2.0 was designed from the start to accomodate campaigns of a certain size/length. It "scales" throughout the campaign using the Act I/II structure. The balance of the game hinges on the fact that the players will never have access to every skill available to their character, and have only so many opportunities to equip their character from a randomised pool.
If the Delve continued past the 6th stage, the game would become imbalanced in favour of the players. Essentially, the monsters can only get tougher by adding wounds ... they're stuck with their Act II cards and abilities. The players could essentially continue optimising their equipment and completing their skill set, which I think would ultimately make it easier for them as it goes on.
Again, I hear what you are saying, and it's something I'd love it to happen, but I can't think of a practical way for it to be provided to us.
Very good points.
I think the progressions for heroes rewards and Exp would have to be slowed down for sure, currently the heroes improve very quickly. And maybe you could start multi-classing after a while to keep it the Xp gain interesting.
The monsters could also get health and special effect armor bonuses but I think you could achieve a level of higher difficulty just by increasing the number of monsters that spawn.
This sounds like a fun game design project