First, there is no definition of what 9.975 really is. Even if you went by the technical manuals (which is pointless), they would still say less than the 21,000 that you say.
Second, while a velocity might be sustainable, that does not make it efficient. With a scarcity of resources, they would be unlikely to sustain such speeds.
Third, they would definitely need to make stops to attain resources. It is unlikely that such a ship could ever be meant to make a voyage meant to last even a few years without refueling.
Actually, Paris stated in the episode the 37's:
Warp 9.9, which in your terms would be 4 billion miles per second.
Sustainable velocity implies that there is enough resources to maintain that speed for LONG periods of time without any danger of resource diminishing (otherwise, it wouldn't be called 'sustainable').
I agree that they would have to make stops to attain resources... but practically all Federation ships have enough technology to park in any Solar system, draw power from the star, use that for replication of other goods, and use asteroids and raw matter in general to convert into energy (or simply use transporters for the process).
And it was also mentioned in one of the episodes of Voyager in Season 1 (when Tuvok was with alien species kids who aged in reverse) by Janeway that Voyager's Warp core can go on for 3 years without re-filling.
Of course... the writers never even bothered to adhere to these notions because ALL of it would easily kill off any attempt at them making the ship look like its in despair - idiots - I would have preferred to see the crew implement their technology to the fullest and you know... problem solve.