I am no expert in finance, but my guess is any decline is likely short term, not long term.
And I base this assertion on the tech companies. Tesla will soon be the largest company in the world BY FAR. Microsoft has a multi-billion dollar project in the works.
There are just way too many tech giants in the US for any serious long-term decline.
PS: Trump has the power to affect things, but maybe not as much power as you suggest in this OP. He's not a dictator as there are massive corporations affecting the economy. Not just Trump and his traiffs.