That trade off is made by evolution. I don't see why scientists in a lab would make that trade off in a gain of function analysis - you can just make it more infectious and keep (likely increase) the level of virulence.
Since evolution isn't involved in the proposed mechanism, I see no reason why such a trade-off would occur.
I think you presume a level of mastery over artificially engineered viruses that just doesn't exist. Is it obvious before a modification is made to a virus strand whether it will be more or less virulent? Besides, engineering for virulence can easily get yourself killed. Furthermore it didn't necessarily have to be engineered for nefarious purposes. It could have been an experimental virus that accidentally escaped and that's all.
Perhaps depends on your goals. Virulent enough to cause global panic and show how a totalitarian state can respond better to a pandemic than democratic stated. But not virulent enough to actually kill everyone.
Similarly SARS and MERS were perhaps too virulent and could thus easily be controlled by screening for symptoms, while the new coronavirus can spread without symptoms, which could possibly be attributed to the lower virulence.
Now this sounds exactly like something Donald Trump has been saying all this time, and such conspiracy theories don't contribute much to the discussion.
If thousands of deaths in China, a complete lockdown that hit the economy incredibly hard and perhaps more crucially, the total breakdown and potential reshaping of the global supply chain, which China has been benefitting from massively, is something that the Chinese gov wants to to achieve, then such conspiracy theories might hold water. I think it's plain to see how nonsensical they are.
Also, nobody made the US look bad but themselves. This is obvious if you compare COVID responses in countries like Germany with those in the US.
Then it was truly unavoidable for the us to keep getting worse and worse, for some reason more in states full of people who think masks are mind control (a total mystery that we will never understand), while other countries are seeing things get better.
It's not that mysterious how tribalism, fear, and the discomfort of admitting being wrong, just to name a few contributing factors, can result in people holding irrational beliefs.
They're not a different species, they're people. If you give into the urge to treat them as "other", as inferior, or as a different "tribe" to be defeated you're not as different from them with their ridiculous beliefs as you might like to think.
That's not completely true. Some of those pressures still apply to artificial viruses once they're released. Highly dangerous viruses get quarantined, and viruses that produce symptoms quickly (and don't transmit asymptomatically, or have a shorter window) are more obvious. Both of those properties reduce real-world transmissability, which is all that actually matters.
I'd argue that SARS-CoV-2 is very far from the Pareto optimal trade-off in infectivity and virulence. There are viruses that are both more virulent and more infectious.
See "A transmission-virulence evolutionary trade-off explains attenuation of HIV-1 in Uganda" https://elifesciences.org/articles/20492