For a bit of context, I have been a Debian user for almost 30 years now. Mostly using testing for desktop / workstation systems, and stable on servers and containers. Debian is comfortable and provides me with stability where I need and cutting-edge where I want. It mostly “just works” with great defaults for everything, and it’s easy to customise where I desire.
With that out of the way: you’re not wrong. In fact, the https://www.debian.org/devel/testing#faq describes situations where these kinds of breakages could happen.
That said, this is exceedingly rare if not nearly unheard of, and I can always pull packages from sid or experimental if I need (like I do Mesa).
Edit to add: for anyone interested in trying out Debian testing, please check out the Wiki: wiki.debian.org/DebianTesting