In my opinion, becoming a full couple in the middle of a story actually means more danger for the side characters in a straight-up ecchi romcom. Since "will they-won't they" is no longer applied to them, authors often would write them out, or at least severely reduce their panelspace.
Now, if it was a romance drama, side characters becoming a couple would most likely guarantee their longevity, since they'd become a "beta couple" and play a mentor role for the "prime couple". They could even get whole plot arcs to themselves.
That depends on the author, Konoka and Setsuna got a good ending while Risa and Mio got practically written out of To-Love-Ru.
