chat.openai.com has about 2B visitors per month with an average visit duration of 7 minutes (
https://www.similarweb.com/website/chat.openai.com/). It is not unreasonable to expect that every visit costs OpenAI about $0.10 dollars; which they pay directly to Microsoft. In effect, this means that Microsoft's revenue is 200 million per month or 2.4B per year if this continues like this. That's already equal to 1% of Microsoft's total revenue of 2022. Note that this is chat.openai.com only but I can imagine that the API also has quite some traffic. Oh and wait, on top of this Microsoft has a 49% ownership of OpenAI and OpenAI is still the best LLM in town, so it's not unreasonable to expect further growth.
> [...], but it's not at all clear that their lead is going to stick long enough to turn it into anything
People are saying that since January, but here we are in July and GPT-4 is still the best model. While everybody is catching up, they have (1) the processes in place to come up with new ideas and get them working (2) a head-start of many months (note that GPT-4 was basically done in Sep 2022).