Yea, it's usually easier to run a campaign against the status quo than for a specific alternative since, by specifying an alternative, you're reducing generality and reach.
In this specific case, though, greater specificity could help anti-YTZ groups. Pretty big chunks of the electorate don't attach any huge costs to having Porter on the Island. There was a lot of FUD spewed about the airport early last decade, and now about the expansion, but most of it was pretty obviously not true. Since fairly few people seem to see anything wrong with the status quo it'll be harder to get traction on a negative campaign.
OTOH, if island groups came out with some sort of alternative (broadly imagined), the debate could be framed in terms of the opportunity cost of this airport. Rather than holding up those stupid placards shaped like the C-Series' turbines (nobody cares about turbine diameter..), protestors could say 'we're missing out on X-billion of development activity and tax revenue which would come from building on these areas" or the benefits which would come from more parkland.
