I challenge the basic premise of this question as, until there are some significant software and firmware updates for the Mini, these two drones aren’t even in the same category. The Spark is a very capable, consumer level action, selfie, travel drone. It is small and sturdy, but most significantly for this conversation, it has Active Track and you don’t need an RC to control it. Heck, you don’t even need a smartphone. Pull it out, power on, wait a couple of seconds and with the double tap of the power button she’s airborne and taking Active Track videos or stills. If you want some “Quick Shots” connect to your phone and you’re off and running. The Mini . . . not so much.
The Mini (also 2.7k) on the other hand is very capable entry level cinematic drone for people who want to cut their teeth on movie making. It has a superior gimbal for image stability, a longer range and longer flight times. It will not, however, follow you or a friend on a skateboard, or an a hike or along the beach, for instance. Setting up for flight takes a little more doing for the Mini, so you can pretty much forget grabbing that selfie with some friends that have less of a tolerance for waiting around for you to break out your Mini, unfold the legs, break out the RC, plug in the phone, power the controller, bind the RC to the drone and get the Mini airborne and positioned for the shot, and then you hide the RC from being visible in the shot while still hitting the camera trigger.
For US residents, the fact that you don’t need to register it really won’t be of any real significance for at least another 3+ years from now when RID kicks in. You can be certain that by then, you will have replaced your Mini with something else at least once.
Can the Mini replace the Spark at some point in time: Yes. For now let’s see if we can get a firmware update that allows self unlocking and then see if DJI wants to make this the travel drone that can replace the Spark.
In should add that I have both of these drones and love them both. I got some great video with my Mini yesterday, would wouldn’t dream of bringing it on vacation with me with its current capabilities.