Basically your only other option is to find the keys for each BluRay you own yourself. I did go through the hoops a while ago and wrote it up: https://www.bennee.com/~alex/blog/2011/04/18/playing-blu-ray-under-linux/#playing-blu-ray-under-linux

However it's a pain sourcing the encryption keys you need for each disk. While I work hard to prefer FLOSS apps over their propriety equivalents in this case I'm happy to pay the small fee for a perpetual licence of MakeMKV.

It's not really possible to make an open source equivalent to MakeMKV. That would mean anyone could see the decryption key and it would be revoked immediately.

I don't think there's any better alternative currently, but maybe something can be built based on the recent AACSv2 exploits.

https://media.ccc.de/v/37c3-12296-full_aacsess_exposing_and_exploiting_aacsv2_uhd_drm_for_your_viewing_pleasure

Commenting to indicate my shared interest in this (despite the other comments suggesting the unlikelihood of such an option emerging).