I'm thinking that this is unlikely, and the brand seems to be doing well where I am, but as I know nothing about the auto industry, really, let's say it does happen and Mitsu pulls out of the North American market tomorrow.
This is still a world car, available in many markets. It has still been sold in some numbers here, and there are parts numbers for everything. Seems to me that there is/will be aftermarket versions of most parts available from the typical big parts stores. Sure, you will have a harder time finding a Mirage at a wrecker (Ford escorts were good for this), but basic parts are likely to be no problem at all. The US, especially, seems to have a really good network of auto supply - with parts available for just about every antique and rare auto every made. I think the Mirage, being made in large numbers and sold all over the world would not be difficult at all.
__________________________________________
View my fuel log 2014 Mirage SE 1.2 manual: 45.0 mpg (US) ... 19.1 km/L ... 5.2 L/100 km ... 54.1 mpg (Imp)