Take a look at the Linux distro timeline:
https://upload.wikimedia.org/wikipedia/commons/1/1b/Linux_Distribution_Timeline.svg
It’s massive and complex. Yes, there are some strong mainlines and some have already died or been combined with others to continue as a consolidated project. But here’s the thing with Linux distro; as long as there are folks out there who want or need something different and/or specific and have the means to create & build it, they will.
It is the double-edged sword of FOSS. It is discouraging yet also the saving-grace of Linux.
To try and make sense of all the spaghetti lines of all the different distros and try to determine which should stay or go or be combined is a near impossible task because everyone has their own opinions, personal preferences and requirements.
What I would suggest for anyone is; if you know of one or a few distros that you really love and want to remain for the foreseeable future, then use those distros, share your great experiences with them and support them by either contributing to those projects through donations or by helping to fix known problems. That is the best way to help ensure your favorite distros stay strong, IMO.
Mint is not going anywhere any time soon. heaven forbid, if something drastic should happen and the core dev team/community should abandon the project, I would suspect (and hope) others would step in to keep Mint alive. It remains one of my favorite and currently one of the most popular distros and I sincerely believe it will power on through this rough patch.