Here's my two cents on the issue as I see it.
On the one hand, the 8chan's moderation looks to have finally shown its true colors in that they do not approve/like /leftypol/'s presence on their site. I don't know if that has been the prevailing opinion for a while now or if that's a semi-recent development, because I've only seen relatively cordial relations between site staff and board up until now (even so far as /leftypol/ being among the first boards addressed when dealing with the furry spammer a few months back). Regardless, there is a good chance that these attitudes will only get worse as time passes. That's not even to touch on accusations of data/IP collection and such. Moving to our own site like this would give us greater self determination as well as potential greater visibility through not being blocked in google searches.
On the other hand, migrating the population here will not be easy, and we are likely to lose a fair number of users in the process (especially those who are heavy cross-boarders). By leaving, and I make no attempt to hide the petty nature of saying so, we risk being seen as having "lost" when it comes to our holdout on "relevant" chans. On that note, you probably should be certain too that, if /leftypol/ is effectively closed and traffic redirected here, no other successor boards are left on 8chan to try and suck up fence-sitters who are likely to cave in to familiarity when presented with the proposition that they adopt a new community platform. There's also the issue of Bunkerchan needing to be overhauled before it should realistically be taking in people, ideally to make it as consistent with the layout of the other chans as possible for user adjustment ease.
Perhaps my biggest concern though is that by separating ourselves further from the other chans, we risk losing a sense of "imageboard culture." A sizable chunk of the people who end up browsing /leftypol/ are people who stumble in from other boards because "it's on the same site, might as well take a look." If we move off-site, there is a high likelihood that those sort of population sources will outright refuse to go somewhere they believe to be an obscure non-starter. We'll still get the people who seek us out, but that is dependent on us maintaining and expanding a sizable reputation through all manner of channels. Being on 8chan and being limited by the site's reputation in some ways filters out those who aren't going to have the stomach for standard imageboard fair; the uber-idpol redditors and twitter normies who happen to stumble on our board don't stick around too long. If we have our own site, we risk a reversal of this issue: the typical imageboard population (at least the center-to-leftist part) shys away and we're left with the previously filtered populace.