It depends how common the niche is and how accessible the body-plan is in the genetic search-space.
We do see convergent evolution into the "tree" and "crab" body-plans. Dozens of unrelated species have evolved along similar lines, filling similar niches by evolving similar forms.
There were long-necked animals in the past -- a well-known type of dinosaur comes to mind -- but the niche isn't so large or successful as the "eat a bunch of grass" niche.
We do see convergent evolution into the "tree" and "crab" body-plans. Dozens of unrelated species have evolved along similar lines, filling similar niches by evolving similar forms.
There were long-necked animals in the past -- a well-known type of dinosaur comes to mind -- but the niche isn't so large or successful as the "eat a bunch of grass" niche.