It depends on how fast money is created versus how fast the growth in goods and services is. If goods and services grow faster than the money supply grows, then prices decline over time. This is really painful to people who have borrowed money and have to do more work to pay it off. If the money supply grows at the same rate that goods and services grow then prices are stable. If the money supply grows faster, we have inflation which we are currently experiencing. If the FED can rein the US money supply growth back to levels closer to the growth in goods and services, then it can go on indefinitely.

Why would it not be sustainable though?

It can be, though the context of how the money is created matters.

Nailing down exactly what money is is a good starting point. Most economists define money by its functions:

• Money is a medium of exchange - you can buy things with it instead of bartering.
• Money is a store of value - you can earn money today and buy something with it tomorrow or next year.
• Money is a unit of account - you can use it to express the price of goods, and thus allow accounting.
• Money is a standard of deferred payment - you can use money to express the idea that someone owes you X money by Y date, rather than needing all aspects of the transaction to happen concurrently.

Keeping the monetary economy stable is broadly the process supporting those functions.

The management of the money supply primarily impacts the role of money as a store of value (though that in turn has implications on the other roles).
Importantly, money need not be a perfect store of value - and no currency ever has been. But it does need to be relatively stable.
If money is created without regards to managing this, it can cause inflation. If money is removed from the money supply without attention to this, it can cause deflation.

Specific forms of money creation, however, can have different degrees of impact. The example you gave - a loan has a diminished impact because the money is created in pair with a debt to be paid: in effect, an equal amount of negative money is created in the form of the debt. There will still be some impact - people value today money more than tomorrow money. This idea is called the "present value of future money," and the difference between the current value of the money and the value of that same money in the future is called the "discount rate." The nuts and bolts can be more complicated, but for the 1000 foot view, you can consider the impact of a loan on the money supply to be more closely related to the discount rate than the raw value of the loan.

This gets at the idea that there are different kinds of money supplies. Economics has terms for these kinds of currency. M0 is actual, hard currency. It's issued by the government only, and it's the most disruptive if produced recklessly. Pointedly, sound government's don't do that: that's why, for example, the Federal Reserve generally addresses the money supply by issuing treasuries rather making additional US dollars.

M1 is fairly close related: it's M0 plus most of what would seem like a normal bank account to most people. M2 also includes money market accounts and similar. Money creation here should still be fairly well controlled.

M3 & M4 starts working in terms of sovereign debt and commercial paper. This tends to be what we're talking about when people panic about "The Fed is creating money!1!!1!." But all of this supply is created in terms of debt obligations. Like your loan example, the impact is diminished by the negative value of the debt.

i wouldnt say so, and to explain it to you simply i would give you this example: if you have 10 slices of pizza to divide among 10 people, the value of each slice would be one slice per person. if instead for some reason, there are now only 5 slices, each slice now has double the value it had before. conversely, if there were 20 slices instead of 10, the value of each slice would be lesser because there are more slices for everyone. the amount of slices creates the value of each slice, and in the same way the amount of money creates the value for money, and if everyone creates as much money as they wish, there would eventually be so much of money that it would lose most, if not all of it’s value. the scarcity of supply of money is what basically gives it its value