Users: Profit through deflationary design (demand higher than supply). Most currencies are inflationary by nature, while Bitcoin is designed to be the opposite (the first phase aimed for growth is inflationary, but this pressure decreases substantially every halving).
Developers: Profit from being a user, as well as from the power to drive additional demand by improving the ecosystem (make it easier to use, more powerful or more efficient), and lastly by way of bounty/salary/donation based on how well they are achieving #2.
Miners: Arbitrage between cost of electricity and price of crypto, transaction fees mined, block reward, and finally from being a user.
The funny money some exchanges offer to make moving fiat easier or faster between other exchanges is pretty irrelevant. As long as tether is not committing massive fraud by minting billions to buy crypto on a fraudulent basis, it doesn’t matter what token people use to move their fiat. Tether could explode tomorrow or in decades from now, but I don’t see them bringing down crypto longterm, just like I don’t see an Elon tweet having the potential for long term impact.
I would certainly never touch Tether, and anyone who does is taking a not insignificant risk of getting burnt if there is no-one willing to buy their tether when they want to sell (Tethers are not and have not been redeemable for dollars from Tether corp for a long time).