Who is going to watch a show they already know the ending to? CBS would want a show to last for years and years... if you already know who she is going to end up with, and in fact HOW she meets him... what's the point? It seems like a small number of people would be interested in such pot hole filling story telling.
People are struggling to stick with THIS series...
Well.. who was going to watch the developing romance between Ted and Robin after they flat out told us in the first episode she wasn't going to be the mother?
It's all in the delivery.. i loved the relationship between them even though i knew for sure they'd split up sooner or later (that always happens with lead characters who enter a romantic relationship, see Ross and Rachel from Friends) but they put the unmovable block between them that they will never end up together for good in the show.
This is why i think this spin off could work but it would need one heck of a writing team to keep people interested in a show where they knew that some events can't work out because she needs to end up with Ted.. with a good writing team you might end up with solid 5-6 seasons (the amount of seasons HIMYM should have been before they had her meet Ted).