The policy is that it's committed to the discretion of Ops, unless someone in senior management intervenes, e.g. a US Senator or Cabinet Secy is delayed.
You should presume that the flight will not be held and that if you arrive at the gate 1 second past T-30, that your seat will have been given to somebody else.
Flights might be held where the flight isn't overbooked and at risk of oversale, the WX is better than expected, the crew doesn't have timing problems and a host of other factors.
While 50 minutes is doable at IAD, there is no way I would attempt a 20-minute connection (remember, T-30), unless I absolutely had to.
The real question is what is Plan B? If you noshow for the xIAD, what are your options? Not many.