I guess we don’t know the details, but with Trump exploring raising tax rates for foreigners: Donald Trump threatens to double tax rates for foreign nationals and companies synthetic ETFs (and UCITS for non-US) might start making a lot more sense if withholding rates are going up.
Switzerland would be targeted on paper as a country that’s implementing the OECD deal: https://www.efd.admin.ch/en/implementation-oecd-minimum-tax-rate-switzerland (maybe they’ll mostly target big EU countries instead and forget about Switzerland)
edit: looks like he wants to apply 26 U.S. Code § 891 - Doubling of rates of tax on citizens and corporations of certain foreign countries | U.S. Code | US Law | LII / Legal Information Institute which explicitly reference the part of the tax code for tax withholding of US stock ( § 871)
edit2: and since the US-CH tax treaty explicitly references the 15% rate (and I assume other treaties are similarly worded), it could only be implemented by also terminating the tax treaty (which is not unheard of, the most recent one that was terminated by the US is the US-HU treaty). So that would be fun… (it would mean 60% withholding)