Agreed, Bolas' gambit pile-up still isn't coming together for me. Besides, he could just summon his freaking army to another plane if given enough time and/or mana. Oldwalkers used to do that sort of thing all the time back in the day, and Bolas should still be more powerful now than the average planeswalker was before the Mending. ...
It's not an issue of power, it's an issue of whether it's magically doable in a post-mending multiverse. And it's not doable. Except that one time in Agents Of Artifice. And now portals are apparently doable too. Whenever it would be convenient for the story really. So yeah, let's not kid ourselves, it's going to become increasingly less "impossible" as we go along.
My head canon has always been that the Mending didn't actually make interplanar travel impossible - that would make the multiverse a little too limited and the walkers a little too special, the Mending just changed the methods required or upped the mana cost.
Planeswalkers in AoA summon creatures from other planes all the time, it's just explained or at least implied that their summonings don't stick around permanently, at least in Jace's case. But I'd be pretty shocked if Bolas couldn't do it the old school way and just dump people on another plane. He's infinitely more skilled at pretty much everything than Jace, so at least for him, it should be doable. If they want this to be completely impossible post-Mending, they should have explained it. If they don't establish it in advance, it's a plot hole as far as I'm concerned. I've said this before, but it really bothers me that some of the rules of how the multiverse works now still feel fuzzy
ten years after the Mending. Can planes still touch and create natural connections between each other? Is it still possible with enough power and skill to summon an army to another plane and keep it there? Can people still summon aether copies of creatures? What about mana burn? I don't want any of this to change because I think there is absolutely no reason for it and it would just make the multiverse more boring. They can't use the Mending as a lazy excuse for everything. Mana burn in particular would be where I draw the line and say, no, I'm
not buying the Mending changed it. At least the other things I mentioned have something to do with the fabric of the multiverse and the Blind Eternities. The fact that conventional planar portal technology doesn't work anymore was only established in-text when Liliana discovered the
Planar Bridge, so it effectively didn't matter anymore. The exception to the rule was established together with the rule we didn't know for sure existed. But yeah, it's pretty much as you say, the New Era of Storytelling is all about what's convenient for Creative, not about committing to something and sticking with it. All this fuzziness and neglect of the rules of the setting together with the way they write magic and planeswalkers these days feels like they've forgotten they're writing for Magic: The Gathering. Apparently the 'core identity' of a thing only matters when Marketing dictates that it does.