If you want reasoning capabilities that Open Source hasn't matched before today, and I'm guessing just got blown out of the water on again today... there's no reason to bother.
Or OpenAI's usage policy limits reasons (either because of your direct use, or because of the potential scope of use you want to support for downstream users of your GPT, etc.) Yes, OpenAI's model is the most powerful around. Yes, it would be foolish not to take advantage of it, if you can, in your custom tools that depend on some LLM. Depending on your use, it may not make sense to be fully and exclusively dependent on OpenAI, though.
If things change down the line the fact you invested 5 minutes into writing a prompt isn't going to be such a huge loss anyways, absolutely no reason to roll your own on this.
If things change down the road such that your tool (or a major potential downstream market for your tool) is outside of OpenAI's usage policies, the fact that you invested even a few developer-weeks into combining the existing open source tooling to support running your workload either against OpenAI's models or a toolchain consisting of one or more open source models (including a multimodal toolchain tied into image generation models, if that's your thing) with RAG, etc., is going to be a win.
If it doesn't, maybe its a wasted-effort loss, but there's lots of other ways it could be a win, too.
> If you want to be independent for academic/personal reasons, sure you can.
If your goal is to be in business, or to get some sort of reach, or anything other than "have fun tinkering"... wasting developer weeks on "could be a win" is how to fail.