Also, Zuck can point to us feddies not wanting to federate with him, and say “see? Interoperability is pointless, even the geeks don’t want it”. Which is oddly accurate…
I think the easiest counter-argument here is healthy disagreement.
Being exposed to multiple opinions is undoubtedly important and is far, far better for us all in the long run than only limiting ourselves to only those opinions and views we already share or at least like, but having an option to wall somebody off on an Internet platform has its benefits, too, like not actually wasting your time in endless and fruitless arguments. As great as it would for everyone to be able to have a healthy and productive conversation about the differences in their views, it simply isn't wise to honestly expect that from everyone.
Besides, having two opposing ideas communicate on the same platform is not what the fediverse is for - not exclusively for sure. It's the freedom to self-host and self-regulate places dedicated to specific things to various degrees: lemmy.world, for instance, is wide and large and encompasses many things at once, and has an option to federate and communicate with smaller, more niche communities and vise versa, while letting the users open a single account with either.
Otherwise it's just the old Facebook formula of encouraging opposing views to constantly clash for the sake of engagement. That's just not real, not healthy, and only exists for the purpose of being some sort of KPI in a corporation perpetually hungry for money and influence. So yeah, we don't want that.