*OK, outside my lanes here as a sailor, and open for correction...my coffee index is not high enough yet today*
Bringing in the Scorpion wouldn't so much be a 'niche' fleet, it'd be able to replace some of what we already have.
Think in terms of replacing the Hawk and Tutor fleets.
Interweb info tells me that the Hawks have been in service since 2000, with a fleet of between 16 and 22 airframes.
Tutors have been in service since 1962, only flying with the Snowbirds now, with 24 airframes.
Suppose we took those 40-46 airframes, combined into one airframe (Scorpion.)
There's enough life in the Hawks that they could be sold off for some cost recovery/offset, you have a standardized training and flight display team airframe, and while it's not a fighter, the Scorpion can carry out some air-to-mud training, along with ISR.
Your personnel training/qualification load goes down because you are reducing the types of airframes. Your parts inventory can go down because we're no longer trying to keep 54 year old aircraft in the air.
If we were looking at a 'show the flag' capability with basic air-to-mud ability, the Scorpion could do that too, so maybe an extra 16-20 for a single squadron to 'bridge the gap'?
[\cynicism]
Or, if we look at the "Interim buy" of the Super Hornets as being all we get for fighters for a long time....then a cynical person could suggest that having the single squadron of SH meets our 'basic' Air Defence requirements (and we can still use a 6-pack of them for expeditionary uses if necessary), so let's just replace all the other 188's with Scorpions, even though they are not air-to-air capable and pretend that our missions are covered off because we have enough airframes to do it.
If we bought 150 Scorpions for the Training/Air Demo/Show Flag roles, and maintained 20 Super Hornets for real Air Defence and Expeditionary Missions, that'd be a HUGE cost savings, an 'out' for the government of the day. All they'd have to do would be to clearly define the Expeditionary role requirement and North American Air Defence roles in a new white paper.
They could point at the numbers and say "Look, new planes" and the average member of the general public will nod and accept that the fancy new jets meet Canada's needs. Even though they don't.
[/cynicism]