That depends are you getting the information you need?
Are your business analysts/architects even able to answer questions such as, how many net new users, concurrent users, able to summarize the typical workload? Back in the day and i'm only in my early 40's, this stuff used to be well defined. We used to have large documents which go down to the level of expected network load. So its either as you said, its too difficult given the diversity of the systems or they just don't know how to do their jobs anymore. I honestly think its about a 20/80 split. Yes the environments are more difficult to manage, but BA's/architects haven't adapted or frankly just don't care.
BA's can't give me any information which would help me forsee or estimate how much load a project/change is going to have on the environment. So when i'm asked if we need new hardware, I just usually tell them to make sure they plan a proper load test and be prepared to spend money.
In my company, its my job to make sure lights on runs well and highlight any issues related to capacity. For new projects, then its part of the project team which I may or may not be a part of.
Storage, for us, seems to be the largest constraint, with memory and cpu coming in behind. Since we can't get much information, we just make sure we have all our servers hooked up to a large san so we can quickly provision more space.