So, an engine-only reuse strategy that they hope will break even after 6 to 7 reuses. Engine-only reuse could make sense, so long as no only competitor has a better, cheaper reuse strategy.

To repurpose Musk's frequent analogy, if jet aircraft had to be thrown away after each flight, no one would fly. If only a jet's engines were reused after each flight with everything else discarded, very few would fly. It is only when the entire aircraft is easily reusable that air travel becomes common, and cheap.

Why would ULA target semi-reusable route in the face of a fully reusable competition? There are a few possibilities.

One is that they believe launch frequencies will stay low. If launch frequencies stay at their current rate, developing a fully reusable platform could be seen as a waste of resources. A lot of expenditure for a limited return.

Despite strong suggestions that launch prices seem likely to tumble over the next decade, ULA may not believe this will result in a sharp increase in customers. Betting against an increase in customers in the face of dramatic price decreases violates some very basic tenants of economics, so it seems unlikely to be ULA's reason, but it cannot be completely ruled out.

The other possibility is that ULA find SpaceX's reuse solution to either be too difficult, too expensive, too time consuming, or perhaps a mix of all three with a large slice of Not Invented Here thrown in.

This latter group of reasons would be my guess. In a world without SpaceX, with low launch frequencies and high costs, where everything is expended after a single use, an engine-only reuse plan would make sense. Faced against a competitor with full reuse, ULA's engine-only reuse only makes sense if it's the only reuse path you've been allowed to follow.

This suggests ULA are not targeting engine-only reuse because they believe it's superior, but because they cannot put together a full reuse solution within a reasonable time frame or at the cost limits set by their paymasters.

Needless to say, this scheme seems half baked. They'll likely spend billions, only to be bested by a fully reusable system years before their partially reusable has flown even once. The sooner and more successful SpaceX is at reuse, the more likely ULA's program is to be cancelled. Lockheed and Boeing are not likely to fall victim to the sunk cost fallacy. If SpaceX can prove a wildly superior product, it would not be surprising if Lockheed and Boeing cut and run.