So Labour do it and they become irrelevant for decade or two!
Maybe they do and maybe they don't. The tide is turning and remain would win comfortably if the referendum was re-run today. It's hard not to wonder how much more the needle will continue to move as the Tories stumble drunkenly through the negotiations.
Ultimately, whether you're a pro-Remain or pro-Leave MP/Party, you're going to be alienating somebody. But I think we're past the point that people would simply turn their backs on Labour for a generation in significant numbers if Labour opposed Brexit.
The needle is moving, slowly but surely. The GE showed that people are already pretty sick of Brexit, and would rather their government focused on the day-to-day issues that affect ordinary peoples' lives. What has the government accomplished of any worth for the past 18 months? Not a jot.
My point is that peoples' opinions of a party or its leader are taking into account a breadth of issues that the Tories don't want to give them credit for. They'd rather try and make the next 5-10 years all about Brexit, to the detriment of actually governing domestic policy.
Maybe not right now, maybe in 9-12 months, but I think the point will come that speaking out against Brexit won't be anywhere near to a political death sentence. On the contrary, it will come as a quiet but meaningful sigh of relief to many. That is, assuming things continue to go as badly as they have been so far, which I've no reason to doubt.