No negotiation is ever stand-alone. It's coloured by what's come before and what follows for everyone involved.
So your actions, choices and words will matter down the line
To apply that to Art.50, yes, you could walk away, but that wouldn't mean the end of it, in either the narrow or the broader sense
Narrowly, walking away from the Withdrawal Agt would not only leave many points of regulation and politics in an uncertain legal situation, but would also likely result in extensive litigation for settlement of liabilities
Since you ask about flights (since you will): if there's no-deal, then "UK would envisage granting permission to EU airlines to continue to operate. We would expect EU countries to reciprocate in turn"