Yes, because opening a second front on Eston's borders would totally swing things in the North's favor.
If Darka would fight Talerium, there would be Northern armies sacking Cagilan cities within a month.
I would say the same thing about Eston fighting Coria. We were making great progress on bringing Coria to its knees; Darka and Eston had just sacked their capital, and we were successfully whipsawing them between Barad Falas and the Barad Gardor. I would argue that we'd already be sacking Cagilan cities as of right now if Eston had never signed that treaty in the first place. You were doing fine when it was an open front, so I can't see what you have to fear about opening it up again. Let's face it, the North was far more effective in the war when Coria was in play. Since then all we've done is loot ineffectively and annoy Talerium. That and lose battles to Tara.
That's my perspective of course, and it's clear you feel differently, but given that you and I can disagree so starkly about Coria, doesn't it make sense that you and Jaune could disagree about Talerium to an equal extent?
That's the biggest obstacle facing the anti-CE coalition; diverging viewpoints. We don't all agree on the best way to fight the war, and most realms are still putting their own interests ahead of the greater good.