"...we are already on the path out, so there is no mechanism to prevent that."
This is open to dispute -- in theory. I don't think that Article 50 is regarded as legally terminal, although I'm not sure I'm aware of all the arguments. Still, if nothing else then Article 50(3) talks about "extending" the period between triggering Article 50 and actually leaving. It stands to reason that this extension could be effectively indefinite.
Morally of course is a different matter, and I certainly accept that you'd need a very good reason to back out at this point.
Still, the fact is that, short of revolution, you either accept what you are given no matter how awful it is, or you are given a say. As long as that say is itself not somehow terminal, ie not a "this deal or no deal" question, where what "no deal" means depends on whoever's in charge at the time, then you shouldn't be afraid of such a say. It will probably end up just confirming our departure on favourable terms.
I suppose the ideal referendum as I'd see it would be something like: yes to this deal; no to this deal, and go back to the table; no to this deal, and leave anyway. If phrased that way, two out of three options lead to us leaving, and the third leads to us trying to get a more acceptable deal. Again, I doubt that the Lib Dems have this referendum in mind, but ah well.