To say Garchomp "outclasses" Flygon is a bit of a misunderstanding of the term "outclessing". Flygon has options that Garchomp doesn't have. For Out and out Physical Attackers, Flygon has U-Turn allowing Flygon To Flee against things it does not want to stay in against, while still doing damage to slower foes (Hi Ice Beam Claydol I am looking your way) whereas Garchomp Does not. Another thing that could be tried with Flygon is some sort of Substitute and Flail Setup as Flygon obviously takes no Sandstorm damage, you could potentially get great results out of using Flail to attack opponents... (I honestly don't know how well this would work tho)
Arguably Flygon can be used a bit more defensively than Garchomp if you like as you have Roost Available to Flygon... Although Garchomp's HP and Defenses are overall better, but it has no recovery move other than Rest(I think).
Flygon is not "completely" outclassed, but if you just plan on pounding the opponent with Physical Attacks, than Garchomp is the way you want to go. If you want something that requires a bit more strategy to use, but gives you the benefits of ground immunity and perhaps recovering HP, then Flygon fits your style well.
Bottom line, if you want to use Flygon there is no reason why you shouldn't use Flygon, just keep in mind, that if you set it up certain ways Garchomp could do the same thing it could, and you are probably better off using Garchomp. Up to you.