Well it all depends on how you define the word sustainable doesn't it?
It is 'The Calculus' that views that this sustainable is not a surge. 'The Calculus' follows Triginometry which follows simple Algebra and this follows simple Arithmetic. It is simple arithmetic that states Voyagers cruise speed (which is 3000 light speed) it would take 25 years to complete the journey, but they must have comparable stop-overs to replenish systems.
Throughout the series it was sometimes evident their was something wrong with Voyager's speed- like they had a bad motor that made them fly through jello .
and this shouldn't be forgotten--
This is Janeway's statement in 'Caretaker'--
JANEWAY: We're alone in an uncharted part of the galaxy. We have already made some friends here, and some enemies. We have no idea of the dangers we're going to face, but one thing is clear. Both crews are going to have to work together if we're to survive. That's why Commander Chakotay and I have agreed that this should be one crew. A Starfleet crew. And as the only Starfleet vessel assigned to the Delta Quadrant, we'll continue to follow our directive to seek out new worlds and explore space. But our primary goal is clear. Even at maximum speeds, it would take seventy five years to reach the Federation, but I'm not willing to settle for that. There's another entity like the Caretaker out there somewhere who has the ability to get us there a lot faster. We'll be looking for her, and we'll be looking for wormholes, spatial rifts, or new technologies to help us. Somewhere along this journey, we'll find a way back. Mister Paris, set a course for home.