Sure, situations will be different. Keep the Sabers then. As you say, the SCE fleet comes in all at a whopping four vessels. Keep them until they build more Mulcibers.
Why "until?" They fulfill completely different roles, so why shouldn't they coexist? Use them both, side-by-side.
Well again, depending on the situation, I think I'd be happier seeing the big cargo jet filled with manpower and material landing on my planet as opposed to something delivering less of both. As B'lenna said when she stepped aboard the Achilles, just knowing it was there was a comfort.
Why even think of it in terms of opposition, as though it were a zero-sum game, either one or the other? Starfleet has a lot of resources, and it's a moneyless society. They can afford to operate the new class of ship without having to retire the other.
And not all SCE crews are identical. Like the tools they use, they're tailored for different functions. The
Achilles and its crew are specialized for massive repair and construction projects, your classic Corps of Engineers responsibilities. The
da Vinci crew is something different -- a special team of multidisciplinary troubleshooters who travel around the Federation solving diverse types of problems. They need to be light, lean, fast, and flexible. A small, speedy light cruiser like the
Saber is ideal for that, but it's totally the wrong use for a massive factory ship like the
Achilles. Remember the engineer's maxim, the right tool for the right job.
There's no reason the two classes of ship couldn't function in concert. A problem arises, a special team in a
Saber swoops in and figures out a solution, and if that solution requires a massive construction effort,
then you call in the
Mulciber to sit around and do the long, involved heavy lifting while the
Saber flits away to deal with the next problem somewhere else -- a problem that may demand a sociologist or a linguist or a doctor more than it demands a factory.