The sorts of modifications to the plans and specs you mention - substituting less expensive consoles and deleting rigging for instance - is simply unheard of on projects I and many of my professional colleagues consult on. It does seem to be more rampant in the a/v world though, which I don't do.
I think in places where there are many theatres, consultants, suppliers
etc to choose from; like the USA / UK / EU
etc... this is less of an issue. But there are a lot of new major facilities popping up in countries with a lot of money but no skills, such as in the Gulf and in Asia. For these, a couple of major contractors in all fields seem to be doing the same jobs with each other on these facilities, and the ability to exploit the fact that the client frequently will not employ an independent advisor in the job and take their advice from the same person they buy kit from; is much more prevalent in these builds. I think this is quite possibly for the same reason as in corporate A/V as you mention - just down to the lack of people in the organisation who are able to (A) spec what they want in the first place, instead letting the sales company do it; and (B) spot when they're being played.
I did gigs at 2 newly opened facilities in Asia. Both facilities had very similar project briefs, similar budgets, similar capacities, similar purpose. For all intents and purposes they could have most likely built the same
theatre twice and everyone would have gone home happy. The one thing that was different is how they planned and spec'd the
theatre. One brought in this group of major contractors and said "we want a
theatre. X-big. Y-capacity. Z-theme." The other instead first brought in several members of staff - an HOD LX, HOD Sound + Video, HOD
Stage + Flys, and a
stage manager. They then said "we want a
theatre. X-big, Y-capicity, Z-theme. But also must be A-lights, B-console, C-dimmers, D-speakers, E-sound
desk, F-projectors, G-fly
system...." you get the idea. The end result is one
theatre is drastically better from a technical standpoint than the other. I shouldn't have to explain which. If you assume that they took the same stance on the other systems in the
venue, the overall
venue is probably significantly better all round. It's not cheap to do. Those people should expect $100,000 salaries and so to bring in the team above probably cost them half a million dollars. But in facilities of that size, the savings will have paid for them, and the savings are ongoing too.
In my mind there are 2 main issues that you need to protect against when building a new facility:
1) The design and spec contractor selling you something you don't need, don't want, could do better than,
etc; to maximise their own take.
2) The installation contractor substituting the specified kit with something you don't need, don't want, could do better than,
etc; to maximise their own take.
I appreciate that you say you don't do this and that is great. But it does happen. Particularly in the field of state-run new builds which have to adopt extremely bureaucratic and budget-orientated tendering processes. Some of these companies have big experience in dealing with large government tenders and how to maximise the benefits to their own pocket without a care in the world for the operational effectiveness and/or occupational
safety of the technical departments of the
venue. And I believe that it could be mostly stopped, or at least hugely limited, if more new
build venues would take the time and expense to employ their HODs before the
build begins, have them supervise the spec and install of the building, and have them there to prevent the contractors getting any ideas about pulling a fast one.