...by rendering them technologically "obsolete" due to having been superseded & replaced, or will those models remain on sale at a lower price as has happened with previous-generation iPhones, etc.?
If history is a guide, Apple will continue to introduce new versions of existing models every year to two years. When a new version of a particular model is released, it will either contain a faster version of the same exact processor, or a newer processor that is similarly a speed bump over the older processor.
This site tracks how often Apple upgrades their models:
This page provides a product summary for each Apple model. The intent is to provide our best recommendations regarding current product cycles, and to provide a summary of currently available rumors for each model. This page is based on rumors and speculation and we provide no guarantee to its...
buyersguide.macrumors.com
The very next generation of Apple Silicon processors are slated to go into Apple's iMacs. Rumor is that they will be significantly faster than the M1 processors in the mini, MBP and MBA. But, that's how it's always been; iMacs are desktop computers more designed for speed rather than low power running to extend battery life (an iMac is always plugged into the wall) or for low heat generation (there is plenty of room for cooling in a desktop.)
So it seems unlikely that there will be any changes whatsoever to the mini, MBA, and MBP contemporaneous with the release of new Apple Silicon iMacs. The mini, MBA, and MBP were recently upgraded, and they are very fast compared to their competition. I wouldn't expect to see them get an upgrade for over a year from now.
There were rumors that since Apple has ditched Intel, and third party video processing circuitry, for it's own hardware, that they would be passing a huge cost savings on to their users. So far, that hasn't happened. Instead Apple is offering superior performance for the price. My guess is that the new iMacs, therefore, will be priced similarly to the iMacs that they replace. But I expect the new iMacs to be a very significant leap forward in performance.