A couple moving from the United States may have 2 salaries, substantial investments and an apparently comfortable household balance sheet. Appendix FM does not ask whether the household is prosperous in general. It asks whether the application meets the prescribed threshold through a source, period and evidential method which the Rules permit.
That distinction matters where the British citizen has worked in the United States, the US citizen expects to continue working remotely, a UK offer has not yet started or the couple intend to rely on a brokerage or retirement account. The economic value may be real while the amount capable of being counted under the partner rules is materially different.
The financial plan should therefore be settled before resignation, liquidation of investments or selection of an application date. The application form should report the complete position, but the legal case must identify exactly which income or savings satisfy the requirement and why the accompanying US documents are the reasonable equivalent of the specified UK evidence.