The cost can also change dramatically: on top of the higher token costs for Gemini Pro ($1.25/mtok input for 2.5 versus $2/mtok input for 3.1), the newer release also tokenizes images and PDF pages less efficiently by default (>2x token usage per image/page) so you end up paying much much more per request on the newer model.
These are somewhat niche concerns that don't apply to most chat or agentic coding use cases, but they're very real and account for some portion of the traffic that still flows to older Gemini releases.
If this is a deal breaker, then self-hosting is the only solution. Due to the hardware premium, all models hosted by 3rd-parties will be deprecated to make room for newer, better, and more efficient models.
The point is that if you want to build a platform that customers can rely on based on their own schedules of feature development, you need to support models for longer periods of time. For example, OpenAI is still offering older models like gpt4 which was released in 2023 - this gives customers plenty of time to test, experiment and eventually migrate to a newer model if it makes sense.