That's exactly the opposite of how any idea ever should be evaluated.
Better wording would be that Riorden did the best that he could with what he had.
As senior warden and close to his Calling anyway, Riorden chose to take the first swing at the Archdemon. He saw an opportunity amidst battle and seized it knowing that he wouldn't find another chance. Was it a risky move with a low chance of success? Yes. But it was the BEST shot that he had and while he failed to kill the AD, he ensured that his fellow wardens would have a better chance than he did.
Going to Denerim to intercept the AD's army wasn't a dumb move because Denerim was also Ferelden's capital and most populous city. Lose this city and not only do you lose Ferelden's ability to fight the Blight and coordinate it's various arlings and freeholds, but the Darkspawn now have thousands of females with which to make their already large army even bigger.
Waiting for the Orlesian wardens wasn't an option since they'd never make it in time and thus, the Army had to attack and kill the Archdemon to try to end the Blight as quickly as possible.
It's true that having a stupid plan work doesn't validate said-plan. But attack the AD at Denerim wasn't a stupid plan, it was a calculated all-or-nothing gamble since at this point, the united coalition had less to lose by attacking than by not attacking. They effectively chose the lesser of two evils and made the best of it. And the gamble paid off.