Things to do before Big Bang

True, but not easily.

Going by the fact that a clean 3% str cape gave me almost exactly the same damage as a 4 attack PAC, I figure I would need a 15% STR unscrolled shield to replace a 20 attack one. Or a combination, say scrolled for 8 attack and cubed for 9% STR, or scrolled for 12 attack and cubed for 6% str. Possible, but not easy/cheap.
So, an already existing 20 attack shield is still quite valuable. But if one were going to create a new attack shield, a high level one might be a better starting point because it can, with plenty of money and luck, become much stronger.

Which brings me back to my first question, whether it'll be worth it to sell off the 20 attack shield for cash to cube items in the future, - if the price will fall by that time.
Especially when a 3-lined shield can beat it.

If similar products could be created with lower costs (incl. chances of blowing the item up using 30%) prices will drop in relation.

 Spoiler

Usually it's the potentialed/enhanced demigod weapon/equip that challenge the formerly godly items, not items created from the ground up. The cost of creating a godly item thus becomes the original price plus the price of scrolls plus possible losses from item destruction plus cost of cubes. Since the original price drops as more potentialized/enhanced items are created, the cost becomes much lower

I'm not sure if weapons not being used by resistance classes will change in price tho. I doubt people will zergrush into buffed classes when there's three shiny new jobs (and people are sucker for shiny stuff).