There are a ton of old brick and stone buildings from the 1800s that are still going strong in the United States, and appear likely to last another century. On the other hand, there seems to be a lot of buildings thrown up more recently that don't seem be aging as well.
Have building standards in the US declined over time? In what ways, and with what ramifications?
Building standards in the US are constantly being reviewed and revised. It is not a process that is ever really over.
In the US the majority of building codes are developed by the organization known as ICC, this org is a not-for-profit that has voting members that consist of fire service, building officials, industry lobbyists and any other shareholders who have a vested interest in code development. Every three years this org produces something call the model code. For example the model code for building codes is the IBC (international building code). There is also the IFC for fire and IMC for mechanical as well as many others. These model codes are created to establish a standardized minimum standard for codes related to construction and maintenance.
From there the codes are brought to the states. Similar to the process at the ICC level, the states will develop their state codes. They use the model code as their model and make necessary amendments to the code for various reasons. This process includes public comments and multiple revisions for many amendments. Once a state has adopted a code it becomes their own. For example California has the CBC (California Building Code).
In a similar fashion, the state code is then brought to the local level. Typically a local fire department will make recommendations for local amendments to the state code. During this process a local jurisdiction may not adopt codes that are less stringent than the state codes. They may however, adopt more stringent codes for climate, topographical or geographical reasons. For example a city may require each building to provide 500 feet of vegetation clearance from a property due to the structure being in a high wildfire hazard area.
This process of code renewal takes place every three years. Currently city's are using 2016 codes, but will soon, or have already adopted the 2019 codes.
Generally a building is required to remain in compliance with the building code from the year that the structure was built. For example if you live in an unsprinklered building from 1978 that is in compliance, but then current 2019 codes require that a building of that size and capacity be sprinklered, the building will not be required to be sprinklered.
Building codes are driven by two primary factors, safety and cost effectiveness. So buildings made of stone may be really sturdy but they are also expensive. The materials that most building are made of now, single family homes mostly consisting of wood and sheetrock with a stucco wall, are not as sturdy but they have undergone many tests to show that they will provide ample resistance against a fire to allow for the residents to evac and a decent resistance to earthquakes. Also, in commercial buildings lots of buildings are constructed in a type that is called ordinary construction, combustible construction inside and non-combustible on the outside. These have also been shown to also effectively compartmentalize a fire if built to code.
Let me know if this has been helpful at all.