Again, as stated prior by other people, I don't think you grasp what the Software Boutique is about.
It's not 'just another' software outlet, it's specifically curated for Ubuntu Mate - and especially for those that have their non-linux understanding family use it without requiring the 'linux tech' member of the family to intervene or worry. It also came out when GNOME Software was a burning pile of rubbish (way back in 2016).
Again - why it's built on SNAP is due to the fact that the code can be added (thus programs added or removed) without having to do 3-4 code changes (depending on where in the cycle, you would need to hit the upcoming release, the current interim release, and the last 2 LTS releases as well. Or you can just build it as a single SNAP that runs on all of them. Why should the developers (who are doing this for free) spend more time than is necessary.
My opinion on the matter is the boutique should stay, and I understand why it is a snap (which I don't have any personal issue with). If you are worried about security, then simply sudo snap remove software-boutique and your problem is solved, instead of removing the software access that most of us enjoy using, knowing that everything in there will work with our version of Ubuntu Mate.