I would differentiate your scenario in five ways.
First, the risk is higher. When a vulnerability has a public patch, it means the nature of the vulnerability is also public. Sometimes there is even public exploit code. While attackers sometimes find their own vulnerabilities (zero-days), it makes their job a lot easier if they can just use an already-known vulnerability.
Second, if the code was part of the OBS project, then anyone reporting security bugs in the code would report them to OBS. OBS would then be able to quickly release a security update if they decided the issue warranted it. But since Qt is external, security bugs will be reported to Qt, and it’s unlikely anyone from OBS will hear about these reports. So there is no process for the project to respond with quick fixes even for severe issues. That is, unless they have someone watching the list of CVEs - but that seems unlikely.
Third, if the OBS project had written the code, then it would be reasonably likely that someone on the project knew the code well enough to properly maintain it over time. This isn’t always true. Sometimes projects are stuck with huge piles of code that nobody wants to touch, whose author has left the project, or perhaps just forgotten how it works. But it’s true more often than not. In contrast, most projects don’t engage with their dependencies’ source code to such an extent, especially not for something as huge as Qt.
Fourth, on a related note, vulnerabilities often come from newly written code. Probably the biggest reason for this is that security bugs often double as regular bugs, causing crashes or other issues for normal users. This isn’t true for all security bugs: a decent chunk of them have very specific triggering conditions that are essentially impossible to produce by accident. But it’s true for many. If OBS really had left the code untouched for 5 years, then that would probably be because the code worked pretty well. Maybe it’s legacy, maybe it’s not well-maintained, but if the issues it’s causing were really bad, someone would have gone in and fixed it. That in turn reduces the chance of security bugs somewhat. In reality, OBS actually is regularly updating Qt and thus pulling in new code that may not be well tested. (But not regularly enough to be up-to-date with security fixes.)
Fifth… at the risk of sounding entitled, it’s not just about the risk but also about the potential upside. If there are security bugs in OBS-specific code, that’s bad, but all the ways to improve the situation involve doing OBS-specific hard work. With Qt, there is already someone else doing the job of finding and fixing security vulnerabilities; OBS “only” has to pull in the fixes. In practice, of course, it’s more complicated than that. But if we have a system where it’s Hard to pull in security fixes that someone else has found, well, maybe that’s not OBS’s fault, but that does mean it’s a bad system. We should aspire to do better.