The why not let God depose heretic popes?
Aside from that, Pope Innocent III and all the popes and cardinals since Pope Innocent II have all lost their offices due to heresy per Richard Ibranyi - who is himself a screaming example of the reason why the status of popes are not and must never be our concern in the slightest.
OK, I'm not a Sedevacantist (I'm also, to be clear, new to the faith, and not presuming to teach anyone, nor do I have a definitive position beyond "I definitely have concerns about Vatican II").
But I don't see how R + R over Sedevacantism fixes the particular problem of guys like Richard Ibryani. Couldn't an R + R theoretically go nuts in a similar way? "Well, I think all these guys were Popes, I just think every council since Lateran I was a pastoral council on some ridiculous technicality, and I also think since the popes of the last thousand years were flirting with the edges of heresy, we're certainly obligated to ignore anything they say that isn't infallible, which is also everything 'cause Vatican I itself was only pastoral and full of errors" or something. Admittedly, I have this same beef with Sedevacantists that accuse R + R types of "sifting the magisterium of the guys they consider to be the true popes and only accepting what they think is in conformity to Tradition." I mean, the Sedes do that too, they sift their magisterium, decide they don't think it lines up, and thus they decide they don't think those guys are popes at all. What's the difference?
Honestly, the more pertinent thing to me with guys like RI is, while its somewhere on the outer skirts of my plausibility structure that the Church might be left with no *Pope* for a really long time, I can't see any way how you can have no faithful bishops without the gates of Hell prevailing. I mean at that point you're at Protestant level "well, there are some people, somewhere, keeping the true faith" level territory, at which point a visible ecclesiology becomes kinda meaningless.