I'm guessing you have trouble following cause and effect.
We're discussing Sawyer's justification for the durability mechanics, remember? The currency flow in PE can be balanced to do as you say—force the player to make tradeoffs, that is—without any durability mechanics. Therefore, your argument is irrelevant, because tradeoffs are not a raison d'être for the durability mechanic.
Okay, lemme spell it out to you:
1. Many players do not like to buy items. This results in huge sums of money by late game.
2. There is an
option to sink this money into the stronghold
3. If an
option exists, there should be an advantage to it and a disadvantage. Without a second, alternative cash sink, there is no disadvantage to investing in the stronghold for the large fraction of players who do not buy items.
4. Durability provides the second cash sink for those players. For players who like to buy items, there is now a three-way tradeoff, which is even better.
The whole idea, and the fundamental point you might disagree with me on, is that
you should never have enough money to do everything. You then have a choice of where to focus your insufficient quantity of cash.
All that though is just a secondary mechanic to the primary function of durability, which is to provide a reason for multiple characters to have Crafting skills and so construct a tradeoff in the non-combat skills.
The subject of the next update I suspect will be how it gives an incentive to use low-grade weapons and armour. A third tradeoff.
If durability only constructed one of these tradeoffs, it would be a bad mechanic compared to alternatives. That it manages all three makes it a good one. Better to have one mechanic with several objectives than several mechanics with one each.