It would be if the components were actually reusable. It is probably more appropriate to say they are refurbishable.
First of all, dropping expensive aerospace hardware into the ocean is not conducive to reusability. The SRBs need a lot of work to get them back into shape.
Next, the space shuttle main engine needs insanely high chamber pressures to generate the thrust required at the pad and provide the necessary throttleability. That drives its operations cost right out of the realm of reasonable reuseability.
Then there is the reentry profile/thermal protection system. Sure it is reusability, but it is so labor intensive that you would be better off using ablatives.
Shuttle reusability is an example of losing sight of your goal. The goal isn't reusability for the sake of reusability. Reusability has to pay for itself. The real goal you are trying to reach with reusability is reduced operations costs. NASA implemented reusable systems that were more expensive than the expendable systems they were intended to replace, blindly assuming that because they were reusable they would magically be cheaper.
Now they are retreating headlong from reusability, having "proven" that it doesn't work. They have proven anything except that enough bad management can sour even the best ideas.
Almost no system on the shuttle is actually useful moving forward, because almost every one was developed with scant attention paid to how much it would cost to maintain and operate it.
Actually, the SRBs do look useful from the standpoint of being "off-the shelf" and our clearly not having $20-30 billion to start over. Perhaps we won't get to some ideal per-launch cost in a bean-counter's imagination, but it wil be good, reliable, comparatively cheaper than current operations by, in round numbers, a billion or so and get us an alternate space-lift capacity pretty darn quickly.
And as for the Heavy-Lift variant, we certainly have a chance of reducing the cost by redesigning the SRBs to be throw-aways, as will be the rest of the booster, ET, and Disposable Orbiter. Not being man-rated will drastically lower all the costs of the HLV.