While its great that you have a spare $100 a year and the money to invest in Apple hardware to digitally sign Mac builds, not every project will have those resources. Projects that don't have the resources will often publish hashes so that users will be able to check to ensure their download is legitimate before using it. Handbrake does this: https://handbrake.fr/checksums.php
For free, the author could simply GPG sign all releases for all platforms and suggest users verify them. Then, the decision to run untrusted code is the user's decision and bypasses platform monetization.
Then, if suffient and continual contributions of money and expertise permit, official codesigning per platform could be added as another layer (defense-in-depth).
