The main source of complexity isn't the .deb format, but the tooling and infrastructure around the format. It's mired in overcomplexity, and it's very much still in a '90s mindset of building locally with multiple layers of Perl-based tools. If it was rethought to be git-native using docker images or equivalent then it could be of equivalent simplicity to other contemporary systems. When I look at what you can do with the FreeBSD ports and Poudriere or with Homebrew and other systems, I see how much of the complexity has been added incidentally and incrementally, with good intentions, but a radical rethink of the basic workflows are necessary to consolidate and simplify them.

[I used to maintain sbuild and was the author of schroot back in the day]