It's not quite true that Remainers have nothing positive to say about the EU, but certainly in the run-up to the referendum the main tactic was indeed to big up the negatives about leaving. But being in the EU has been good for the UK's economy, it has been good for its trade, good for science, good for technology, good for the environment, and so on and so forth.
If, in spite of all that, people want to leave, they are of course free to choose to do so without being morons. Where the moronic tendencies come is when you scream and rant and rave about treason, treachery, and "Project Fear" whenever anyone points out the rather obvious truth that leaving an institution means losing access to its many benefits; or when you simultaneously insist that the EU has interfered deeply with the UK since we joined yet seem to insist that undoing that interference will be virtually as simple as clicking our fingers and wishing it all away.
The Remain side *did* win the economic argument, by the way: a survey that I linked to a few weeks back demonstrated rather conclusively that a good deal of Remain supporters primarily voted that way for economic reasons, whereas the Leave vote was focused on trying to reclaim full sovereignty *despite* this. Remainers have still won that argument, even if the last couple of years have seen only a minor impact on growth as a result of the referendum. There are barely any economists of note who expect the UK's economy to emerge from, in particular, a No-Deal Brexit with no meaningful impact; study after study has shown that growth will suffer, to one degree or another, if the UK leaves without a proper Withdrawal Agreement.
As a result, Liam Fox is probably right: if No Deal is the only way to secure Brexit, then it is highly likely that Parliament will vote to delay Brexit or even stop it altogether. If you want to change that, then vote for a party that is prepared to deliver such a Brexit and do so without fear of the economic risk; otherwise, your choices are likely to be between accepting a drawn-out transition, that ties the UK to rules it loses any say in for possibly a decade or so, or to stay in the EU and at least have *some* influence as one voice in 28.