Good points. The decline we should be worried about is not in the currency, but in the competitive advantage of the US relative to other countries. If it gets more affordable to do business elsewhere, you'll find fewer and fewer good companies on US financial markets. (Many claim SarbOx has already done significant damage here.) If emerging economies can continue to reduce costs -- by tackling security problems, corruption, red tape, improving education and infrastructure, etc. -- faster than we can, US equities may lose their world-leading position. That's not to say it's a zero-sum game; it's quite possible that all the world's equities markets could do well, and the US could lead without being dominant. But equity investments are more strongly tied to their country of origin than globally traded "real" commodities like timber or oil, so if you think the US might lose some competitive advantage -- like if it got significantly less appealing for valuable workers to immigrate here -- you might want to skip the stock certificates and instead buy the paper they're printed on.
Somewhat paradoxically, U.S. stocks are likely to do better if the U.S. declines relative to other nations. Very few U.S. stocks make the bulk of money in the U.S. Instead, they tend to make most of their revenues abroad (usually from 30-70% for most S&P 500 companies) and pay most of their expenses in the U.S. If the U.S. dollar declines, their expenses drop, their revenues increase, and so their earnings (and hence stock price) go way up. If the U.S. worker declines and can't bargain for the same standard of living they used to have, their wage costs go down, their revenues remain constant, and their stock price goes up.
The nightmare scenario for U.S. stocks would be if international trade goes kaput and war breaks out. Then they'd see the bulk of their market instantly disappear, with a resulting drop in earnings and stock price. But in this case, you should be worrying about how not to get killed, and not where to put your money.