The idea is similar to how GMail hides "spam" and "trash" labels by default.
If there are results, the series list view will show a checkbox at the bottom right corner, "Hide merged and obsoleted series", unless the search topic includes any of "is:merged" or "is:obsolete". The checkbox is checked by default and, if it is checked, the search query automatically does "-is:merged -is:obsolete".
If there are no results, and again the search topic does not include either "is:merged" or "is:obsolete", the series list view will prompt "Do you want to include merged and obsolete series?" and add the appropriate GET parameter to the page.
The idea is similar to how GMail hides "spam" and "trash" labels by default.