Hi RD400,
The ideal spacing would depend on the specifics of the mattress. With latex I would look for gaps that were no more than 3" (and less would be better) while for polyfoam support cores I would look for gaps that were no more than 5" (and again less would be better). There is also more about the different types of foundations or support systems that are generally best for different types of mattresses in the foundation post here.
A slat foundation that is suitable for the mattress you purchase won’t reduce the useful life of a mattress and the weakest link of a mattress is generally the quality/durability of the comfort layers in the mattress … not in the deeper parts of your sleeping system.
Slatted foundations can certainly help with airflow under a mattress and the reason for narrower gaps and/or stronger wood are to improve the strength of the foundation under a mattress so there isn’t any flex or sagging under the mattress.
While these are issues that you won’t find consensus in the industry … you can see my thoughts about the additional risk of a solid surface foundation in post #10 here.
Phoenix