Storymode had originally I think either 6 or 8 Chapters, the rest were added later and are way harder than the initial ones. Your most important rewards (some Class cards) are in the early ones though so you can work on them as you get better decks eventually.

Low ranks are a cointoss, it's extremely easy to get at least 1 powerful deck in this game since you get so many initial packs and tons more from doing the basic stuff. This includes beating every AI Craft on Elite 1 and 2 difficulty, playing 20 private matches against other people without quitting etc. Generally the level of play is abysmal in low ranks even if they did have access to good cards.

Ranked only resets in Master (the overall score gets returned to 0) and Grandmaster (people will be dropped back to beginning of Master every 3 months as new expansion rolls in), it is really fast to climb out of low ranks if you have decent deck since the game gives so much more from wins than you lose from losses during the early ranks. There's also winstreak bonus up to B ranks.