Parliamentarians should vote Mrs May's deal down. This will require them to engage in a bit of long-term thinking.
If the deal goes through, we'll be in virtually the same situation in two years time. A 21-month transition period is nowhere near enough time to negotiate a trade deal, and there are no deals that could ever be negotiated which the UK Government hasn't already rejected (Norway, Ukraine, Canada/Korea, Turkey).
Thus, the UK will be in a permanent customs union with regulatory checks on goods passing between Britain and Northern Ireland, unable to sign our own free trade deals and out of the biggest free trade deal we've ever been part of (the Single Market).
Or, if we manage to get out of that, we'll have paid a £39 billion divorce bill without any trade agreement whatsoever with the European Union, plus a hard border on Ireland would be reimposed.
In other words, we'll have a choice between a permanent customs union (the worst of all worlds according to many) or a no-deal Brexit.
MPs should dare Mrs May to be the Prime Minister responsible for a no-deal Brexit. It's her choice, because there are options on the table that would likely command the support of the majority of the House of Commons, including the Norway/EEA option, which has been on the table from the very beginning of the negotiations.