New stories allow for new characters. As technology and society change, there's immediately room for new characters to fill new roles. Journalism and media are different now than in the 90s, so a story about Peter at the Bugle or MJ doing a TV show allows for characters whose jobs didn't exist thirty years ago.
When the main characters have new developments, that also creates room for new characters. If Peter & MJ had a kid (or suddenly reunited with a three-year old Baby May) it would be logical to introduce babysitters or other young parents who could be part of their social circle.
There are stories that could've been done before given the backgrounds of the characters, but just haven't been explored yet. How often do we see MJ's directors, or Daily Bugle foreign correspondents?
You may have an exaggerated sense of the effects of a supporting character.
Coulson being in the comics doesn't mean he'll be as popular as Deadpool.