You get no argument from me on the economics of the situation.

Currently a spare set of locks for the Westley Richards hand detachable boxlock is about $6300 depending on the exchange rate
(about 10% of the price of the gun) & I would guess that a spare set of H&H type detachable sidelocks would be at least the same % of the gun price or more. A good reason that spare lock sets are not all that common.

However, a back up gun is not always convenient or possible to have along and a backup that costs 10% of the price of their main gun may not be what some people want to finish their hunt with.

Nice things are rarely inexpensive but to some people,the valueprovided is worth the cost.