Weeeellll...... hold on just a minute.
"Successor" doesn't mean it has any relation to the 3900 series, except for that single-stack point and they come from the same manufacturer.
I'm of the opinion that the S&W line lost a lot of sales by not having a single-stack 9/.40/.45 for so long. The Shield series came way after competitors already had a foothold in the single-stack segment of the market.... or maybe more like a beachhead.
And notice one thing: none of that has anything to do with whether or not the Shield is a "Descendant" of the 3913 (which of course it doesn't - the Shield has nothing in common with the 3900 series except for that single-stack configuration....so we're back to where we were.)
So again: the M&P polymer line and its poly predecessors certainly aren't "related" as in design, etc., to the all-metal Gen 1, 2, and 3, but they are "successors" in the sense that they came after the all-metal guns.