Right now, when you can buy a tether at $0.997 and (in theory) sell it back to tether at $1, there's not much scope for profits after costs. So there's not going to be a lot of money pushing the price back up. IOW it's kind of 'expected' that a stablecoin might drift tiny amounts away from its theoretical peg. It's just not worth it. (n.b. even Tether themselves charge a 0.1% fee to redeem their coins)
If/when tether drops below (say) $0.99, and there's no influx of money trying to support it, that's when USDT is in trouble.