> There are cost tradeoffs for the different models when building on them, but for chatgpt, it's less clear to me why one would move between selecting different models.

The same tradeoffs (except cost, because that's roled into the plan not a factor when selecting on the interface) exist on ChatGPT, which is an app built on the underlying model like any other.

So getting rid of models that are stronger in some areas when adding a new one that is cheaper (presuming API costs also reflect cost to provide) has the same kinds of impacts on existing ChatGPT users established usages as it would have on a businesses established apps except that the ChatGPT users don't see a cost savings along with any disruption in how they were used to things working.