The problem of the "time stop" argument then would be... why ARE living things exempt?
What about mana? If you're stopping the mana from leaving, then they're frozen too, right?
Then how do you get more? Will you eventually run out of mana?
1) It's the way the dragons did the magic. If living things were exempt, the dragons with die. As such, the dragons did not do that actually catapaulting their own immortality in the process.
2) Why would the mana be frozen? The mana is created from the bonds the caster makes with the land, not the land itself. There should be no subtraction of mana.
3) See above.
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The Eldrazi, unfortunately, do not have this sort of imagery, or metaphorical "oomph".
Whenever you hear the word Eldrazi / Phyrexia everyone thinks planar domination. So saying that a planes currents existance was to prevent that from happening has immense "oomph".
You can't really use the Eldraze as a "tool" for the plot, because they're less of a "tool" and more of a "Giant Goddamn Sledgehammer" As has been noted, there really is no reason to include - or even mention - the Eldrazi at all.
This is in direct contradiction to your last statement. Eldrazi don't leave an impact; they have a massive impact. Perhaps this is teh reason I'm so confused, because the people fighting against the idea appear to be contradicting themselves. Not to mention that nothing has really been set in stone yet, though that's not anyone's fault it's just another reason I'm confused.
Besides, as far as creating sets go, we should keep the focus on what's interesting about this idea. This plane, this set, is ripe for exploration! Why bring in another set's - canon or otherwise - big, flashy monsters? We already have a plane that is locked in stasis! Who needs Eldrazi? It's not important, it's superfluous, and it comes across as gimmicky for the sake of recognition and attention.
This is where the plot tool comes into effect. This land HAS to be very special if two dragons were to take extreme measures to protect it from the eldrazi. I personally, would simply evacuate the plane until I could come up with a solution to the eldrazi, but the dragons didn't do that. They stopped the entire plane at some great cost; but also for some great gain. Dragons are selfish, but that doesn't make them heartless.