Given how many times the world has been destroyed, the deal with Hel seems pretty short sighted now. Either Loki didn't realize that everyone dying due to the gods unraveling creation would count as dishonorable, or he was hoping that this time the Snarl would break out and devour all the dwarf souls before Hel could collect.
...Unless his real plan was to trick Thor into an unwinnable situation and sow chaos amongst the gods, which I'll admit is pretty in-character.
I do not know about that, but I just realised something. Dwarves do not live forever. I looked it up on D&D wiki and apparently, their natural livespan is between 250 and 450 years (mountain dwarves).
The world is 1184 years old.
Assuming dwarves are fertile until they start being old (188 years), there were at least 7 generations of dwarves, including the current one, probably more. At least 4 of these generations are dead. And that is with dwarves only giving birth just before their 188th year and dying at their 450th year.
If 90% of the dead dwarves died honorably (There would be some that failed to do so, but most would, knowing that if they do not, they will belong to Hel), even Hel cashing in all the current Dwarven souls now may not make her powerful enough to beat all the other gods.
But the longer she waits, the worse it becomes for her. The number of dead keeps increasing, and most of them die honorably.