True, but if the RN wanted to, they could've just kept Louisbourg working as a base or a naval station as well.
I think the point of the WI is to allow the colonials to seize an navy early the Revolution. Louisbourg is too far the colonials to easily seize the ships intact.
However, if the RN does decide to build ships in North America, then it means a lot has changed or may change. Building the ships in North America means that the British either have to official rescind or cease enforcing things like the Iron Act, which prevented manufactures from developing in the colonies. IIRC it was enforced in the 1750s or 1760s to close down an iron foundry in New York; if you're building ships in the colonies, it would far more convinient to have all the raw materials close at hand.
Accordingly, this would represent a big change in British imperial policy of just the sort that would appease enough Americans to avoid the Revolution as we know it. Doubly so, since it would probably divide the mercantile class (further).