When Marvel says "long-term", they simply do not mean that the readers that left will come back... they're thinking at a time when many of those readers are being spoon-fed pablum in an old age home... and the kids/adults of that future era who will be buying the books/reading Spider-Man is who tehy're targetting.... long-term viability of the character.
They felt that the marriage was a means to an end, and they do not want that in their characters, especially the flagship one.
It still doesn't make much sense when they have Ultimate Spider-Man and Marvel Adventures Spider-Man out there for the folk who want an unmarried Peter Parker...and they weren't selling as well as ASM was with a married Spidey before BND.
I understand it's their flagship character in their flagship book and they want him to the ideal that they invision, but they are taking quite the hefty gample by alienating part of the fanbase in an attempt to get there...especially the way they did it.
As for the potential to attract "future readers", the jury is still out on that one. Right now, Spidey is a watered down version of the late 70's early 80's Spidey, and there is now a ceiling placed on what you can do with him. That will hold for a bit until his first serious relationship...then people will inevitably wonder why he hasn't talked of marriage, or why it seems that his relationaships all seem to implode, no matter who the girl happens to be. You can only say "Spider-Man destroys Peter's relationships" so many times before you start to wonder why, of all the women Peter meets, none of them are capable of dealing with it.
It's just kind of redundant.
Before the marriage, it was all up in the air. Peter may get married someday, Peter may not, but we all read it knowing there was potential. Now, they've gone back and said that the marriage NEVER happened (despite me having about 300-400 issues of assorted comics that say otherwise), and that would actually work if they could make us all forget it.
They can't, and now we're all left with this knowledge that Peter made it to the next level of a relationship, only to have it taken away from him because it was a mistake, and now that it has been stated that Spidey will never be married, we're supposed to ignore the ceiling and look to the sky? Easier said than done.
Honestly, I'll put it like this:
If the marriage had never happened (IN OUR WORLD,NOT PETER's), then this would be redundant, and we would have the stories that (supposedly) everyone wants. Of course, Marvel thinks it's the same thing to say it never happened, and we're supposed to run with it. For those of you who can swallow that pill, good for you.
Me, I just can't help but shake the nagging sensation that Marvel threw away something rare and beautiful when they got rid of the marriage...something that was abused and never truly utilized.