It is of course ideal if a single location could provide multiple different audio setups, so that a single visit could produce multiple usable PRIR's. But the reality is that this might not be possible, and/or to reconfigure a single setup to conform to different channel requirements.
For example, at AIX they had side speakers which could be easily moved. So they could be positioned where they ideally should be for either a DD5.1 (+/- 120) or DD7.1 (+/- 90) setup. And it was possible to activate two additional LS/RS speakers (+/- 150) when doing a 7.1 calibration. The speaker connections and movements took manual time and effort, but it was not overly inconvenient. Of course it was then necessary to run an independent second complete calibration process, which of course took more time. The studio was thus required for "two sessions" of use, although we kind of pre-arranged for a big block of time so we were free to do whatever we needed to maximize use of that block for the several of us who were there, and who did have the desire to come away with both a 5.1 and 7.1 PRIR.
But this was not a THX setup, it was DD. Turns out they actually DID have additional THX speakers mounted higher up on side walls, but these weren't part of our intended DD sessions, unless you actually wanted to also get a THX session. If you complicate things with vertical and presence speakers, or different speaker placements or angles, and if it's even possible to use the same single studio but only after physical speaker placement changes, well obviously this significantly complicates things and increases the time requirements and inevitably should also increase cost. That's pretty much the major factor: cost, and what you are willing to pay for what you come away with. And every studio may or may not have any flexibility whatsoever, so what you see is what you get, period.
Again, the real purpose of a PRIR is to capture a specific single listening environment. Like it or not, that Egyptian Theater PRIR session was specifically to capture the sound of the Egyptian Theater, not to use it reconfigured five different ways for other PRIR's if that's simply impossible. So, if this Blackbird Dolby Atmos "miracle" Studio C opportunity is truly possible to arrange for (at a standard cost), then THAT is what we should use it for. If you want some other DTS-X environment as well, see if you can find one of those to measure. That's why you will be able to collect and use multiple PRIR's, to reflect multiple different listening environments which may actually have come from multiple different actual rooms with their own unique equipment setups.
And again, even if you do acquire multiple similar PRIR's, you will almost certainly begin to prefer one or a few over others. The unused ones will still be available as novelties, but it will be clear which PRIR's give you the greatest enjoyment when listening to content through them, and those will inevitably be the ones you set up for use in presets.