The point is that the Circle in Minrathous was built over the Temple of Razikale which in turn may well have been built over something much older. PW confirmed as much in one of his interviews when he says that much of ancient Tevinter was founded old ruins and really that means elven ruins Many of the buildings in Minrathous were constructed with magic and this is apparently all that keeps them standing. If the basis for them was foundations that were constructed in the pre-Veil era, this would account for why Minrathous is literally crumbling away with age. It is possible to get a conversation in the Crossroads with Morrigan in the main game where you can sense the Crossroads is decaying, clearly because of the lack of direct contact with the Fade. Likely why Solas is so concerned about getting rid of the Veil before he loses the Crossroads.
It is possible the same is true of Minrathous, which could mean that the Magisters would actually support Solas' plan considering they are so sure of themselves they probably think they wouldn't perish. Unless, of course, they know the truth of the matter is that everything of old Tevinter was built on the back of elven magic, they never conquered the might of the elven empire and that if the Evanuris should return they should be very afraid indeed (even if they don't instantly perish when the Veil comes down). Which is where Dorian wanting to make them aware of the truth of the matter comes in. Modern Altus are so convinced of their superior magic and bloodlines, they are unaware of how vulnerable they really are. The truth is likely to be found buried in the archives. Nevertheless I am sure there are those Altus who are aware of the truth, just prefer to close their eyes to it. Just like Tevinter responsibility for the Blight. To say "it wasn't us" when their own Chantry is founded on the Canticle of Silence, which clearly points the finger at the activities of the priesthood of the old gods, is splitting hairs to say the least. Corypheus seemed aware of how important elven magic was, which is why he seemed obsessed with getting his hands on as much of it as possible.
I wouldn't be surprised if the Qunari know more on the matter as well. Strengthening the Veil would have reduced the effectiveness of magical generally but would have greatest impact on their long time enemies, the Tevinter Magisters. If it also weakened or even totally destroyed their unassailable capital, from their point of view that would be even better.
Interesting theories, but as of now, they might not necessarily hold true. I quite doubt that some Magisters would support Solas's plans, for a couple of reasons:
1. He's an elf
2. They would never trust the power to raise or lower a Veil to another not of Tevinter blood, not a chance in hell.
In the grand scheme, we also have no idea what human magic was like prior the rising of the Veil, so we do not know for sure if it's erection actually lowered the abilities and power outputs of humans ever after, nor do we know for sure if lowering the Veil would actually increase the abilities of everyone now with magic.
In regards to the Qunari's plan to strengthen the Veil, doing such would not have necessarily lowered the effectiveness of magic, as places even with a stronger Veil hold the same level of effectiveness. Strengthening it in Tevinter might lower the possibility for demons to barge their way through, but that doesn't mean magical constructs in Tevinter will lose their efficacy.
In sum, my point is that just because the raising of the Veil severed the ties of magical constructs anchored both Fade and reality, it does not necessarily equate an actual change in the magical strengths of human and elven mages as well as the effectiveness of magic in general.