I think the investment was a bet that the Stack Overflow model could be replicated in many verticals. If they're able to achieve being the de facto QA site for other professions like they are for programming, I think there are several revenue models that would be possible.

I think you're asking the wrong question. Show me how the need for ROI has hurt the current user experience.

Until the VCs are satisfied with their current ROI (and there's no indication that they are), they'll probably push for more monetization - some of which will probably end up hurting the user experience eventually.

You said "probably" twice in that sentence, which is a clear sign that you're speculating. There's no indication that they're not satisfied with their current ROI either. I'll be a contributing member of SO until I actually see evidence of something (whether it's the VCs or something else) hurting user experience.

VCs care about future ROI, specifically the exit. Stack Overflow needs revenues only to the extent that they enable a big exit. They might not need to be huge if the exit is a sale to a bigger company interested mostly in the traffic (see: YouTube).

Are there signs that the VCs are unhappy with their ROI?

Because the user experience has not yet suffered does not guarantee it won't. I am not saying that the user experience will be ruined, only that it is technically possible. We just have to imagine SO is sold and the new owners put more ads, for example. Do you know if the current investors are happy about the current ROI?

It is technically possible for the earth to be hit by a comet tomorrow, wiping out all life. Just because this hasn't happened in our lifetimes does not guarantee it won't.

Instead of posting comments on the internet, you should be spending time with your loved ones.

Why speculate on what might happen? I can always bail out and stop contributing if something changes and Stack Overflow stops being a valuable resource for me as a developer.