The Downsides of Doing Undergraduate, Masters, PhD at the same institution A bit of background: I attend University A. I have done all of my studying so far at University A. I completed my undergraduate degree in Faculty 1, I will complete my masters in Faculty 2, and I am considering applying to do a masters (track to PhD) in Faculty 3 to study a new field. (I can clarify the field jumping if needed, but not sure if it's necessary) 

I understand that if somebody started out in chemistry, for example, that doing an undergraduate, masters, and a PhD at the same institution could have some career downsides (smaller network, access to the same resources, etc). In this case, it seems clear to me that there is an added benefit of doing a PhD at another institution. 

In my case, however, I have been in multiple faculties within the same institution. If I were to apply and do a masters/PhD in a different faculty at my university (University A) then it's not as clear to me what the potential downsides would be for an academic career after the PhD.

So I am curious what the opportunity cost of pursuing a new field at the same institution is, and ultimately, having finished ones entire academic career at one institution.
Normally the downside of studying at a single university is that the student sees and interacts over the years with fewer ideas since they keep coming from the same people. Once a person gets a degree from a university department, they may have been exposed to a set of viewpoints smaller than what is available if they looked wider. In other words, you can get into a mental rut. 

But that doesn't seem to be the case here since, I expect, that your set of contacts even though in a single university is quite wide. 

But you might, just for fun, talk to a few of your most trusted faculty contacts and ask them whether it is time for you to move on or good to stay. If you are a highly valued student, then there may be a bit of selfishness in their answer, so watch out for that. But if you detect such, then it is surely time to move on. 

Over your career you want to develop and maintain a wide network of collaborators. If you can do that while still based in a single place, then you should be fine.