I think if Marvel has a line or two to try and get new readers, old readers wouldn't feel as defensive when a new character came in. Legacies are also a tricky point, because they do work when done right, they can lose readers if they just feel like a cheap replacement for their favorite character.
I didn't see the Spectacular issues you're mentioning, but I might look for them later. Spiderman is a great gateway to introduce new characters, they should introduce more in his pages and see who catches on. Carol's original run gave her Spidey's entire supporting cast to help ease readers into her. Marvel needs to focus on giving older readers things like Gwenpool (solid writing, fabulous to passable art, new character that brings a different perspective to the Marvel Universe).
I grew up Marvel, I'd love if they were great these days, but I'm not feeling it.