It should be good. So far people day trade. If there would be no stable coins then they would more hold then day trade. Since when selling Bitcoin would have to pay taxes. But. Tether is not the only stable coin. There are plenty and and more and more are coming. Libra soon, .... So when tether goes down will be no changes.
No it would not be good. I don't think tether will collapse but in the off chance that it actually did, then it would be disastrous and even if you hold Bitcoin, your Bitcoin value would most likely decrease anyways because it would turn out that tether ended up being some scam and many people would just get out all together and just invest their money in the stock market.
Libra isn't really going to be a tether competitor, its going to be centralized and who knows if it will actually go live. When they announced it many of the government bodies stopped in and tried to stop it. By the way these meetings go, it could be years before Libra is actually released.