Coming from someone who has been a heavy Armstrong critic, his latest moves have tempered that along with the Cup run of course, but I don't think firing him over Petro would be wise.
As others have said, there are plenty of variables in play with this situation that could result in Petro signing elsewhere. Some are on the team, and Armstrong's side, but others are up to Petro and his agents, family, personal decisions, world events, etc.
If we find he walked over a simple offer or something that could easily been met, we should certainly hold Armstrong accountable as we should any other deals. Winning a Cup should not cloud our judgement completely. But whether Petro stays or goes, it's best to at least let Armstrong ride out the rest of his contract for a few more seasons and see what he does to continue building the team, maybe subsequent moves win us another Cup anyway. Either way his contract is only 3-4 seasons more, so I say unless he goes and starts selling the farm to put us in the basement I would just evaluate making a change or not when his contract is up.