It has nothing with faith. It is exactly a cost benefit equation, while in the world there are both rational and irrationals, we are all agree rationals are far more than irrationals, and if some irrationals wants to hurt other in cost of their money no one can stop them, until they ran out of money. These attacks will be system costs and will be very small comparing system benefits.
You simply keep denying the obvious!
The attacker brings 10 000 satoshis to the table, he receives the same amount in SABU which he spends, then he issues a transaction with a 9500 satoshi fee, which your GT model is unable to overcome. What's the end result?
Attacker 10 000 satoshis in goods bought with SABU, 500 satoshis saved from that UTXO. He is not losing money he is gaining it!
The creditor has -10 000 satoshi in Sabu and..nothing else!
Miners enjoy those fees which make the whole scheme unfold as fast as possible.
The only way you would even be able to make this bulletproof would be to go the way defi loans are created when the users would guarantee with 40 000 satoshis per example but receive only 20 000 satoshis worth of tokens to spend in your network, making the collateral a barrier to profit from the scheme. But! who is going to entrust you with twice the amount of bitcoins just to be able to spend on that amount in a token nobody has heard of when there are hundreds of other solutions out there far cheaper, he could simply switch to a damn altcoin and keep half of the money safely in his pocket not in some really really complicated scheme.
As I told before, there is no central server (except google play or apple store to download the mobile wallet) at all. So every one pay her/his part. If you need a good email service, pay for it, or just use free services.
So either the risk of google disabling my accounts as it doesn't like those mails or pay for a paid email service that would cost me more than the fees.

You're seriously downplaying the disadvantages of your system, I understand that you're trying to defend your model but you should really look at it from the perspective of the customer! What would the average Joe think of this?