If this is really the case, shouldn't it be worded as "most traded cryptocurrency" rather than "most widely used cryptocurrency"? Because as far as I know, though Tether is being widely traded for sure, not only for BTC, but for other alts, but I'm pretty sure Tether is very rarely being used as a method of payment and as a store of value, which actually people use bitcoin for. There's a quite significant difference between "most widely used" and "most widely traded". It really depends on user intent.
Tether thrives and appears to have a virtual life on its own within exchanges, where several virtual operations occur without touching any blockchain, but that's where it ends.
And it doesn't even has its own blockchain, it needs someone else's, and that means paying in the various coins that own each blockchain.
A "honest" Tether wallet would tell you that you need both Tether and a different coin depending on which variant you have. From an user perspective, this is simply unacceptable.
There is also the fact that, Tether price is a fiction that is kept by human intervention, within their theoretical limits. If they stick to their own plan, and nothing extraordinary occurs in the market... And both things are human factors, i hope you know where that could possibly end.
Suffice to say, world's current worst Fiat, also was pegged to the USD in 2004. Pegging is a form of "stable coin", others more subtle are direct/indirect market intervention, either buying/selling or outright freezing the exchange market. All of these are against free market trading.
IIRC it was 1 600 units per USD in 2004. Today, you would need the equivalent of 2 000 000 000 000 for 1 USD. This is what happens when you let a human factor in the equation... Stable coins are build upong promises, because thats the best humans can do, sure you could threaten them with contracts and stuff, but that doesn't remove the fundamental problem: It CAN happen, as it has happened in the past. Tether itself has experienced it, so far it was a small fluctuation, but it might happen again, not so small, because it depends on human trust, not code. And you know what humans can do, given the right circumstances...
But what does it matter what happens within exchanges, if at the end of the day, people end depositing or withdrawing bitcoin? Of course some people use the USD alike for hedge and their trading, but in the end they don't want to keep that risky coin that a human could turn into ruin overnight. If anything, the traders using it are gambling it it won't happen to them during their operation. You know, like an MLM scheme, those few who manage to leave early, make profit...