> Debian moves to slowly [..] My counter argument is that we want to expand Sage's user base [..]
Beyond that - "too slowly" applies only for "Debian stable"; and users that are OK with less stability can use "Debian testing". Usually this is quite bug-free; things only enter testing if it's been in "Debian unstable" bug-free for 5-10 days.
It's also much easier to get your software into Ubuntu, if it's already available in Debian testing/unstable - and that would likely expand your user base quite a lot.
> some of the upstream dependencies of Sage refuse to accept patches [..] [we could] maintain Sage-specific forks of that software [..] swappable with the originals via update-alternatives if possible [..] [or] completely replace and rewrite [it] [..]
Yeah, the situation is complicated. We could try different approaches for each dependency too, and perhaps some of them will change their mind. Debian does (on purpose) make it quite high-cost to maintain forked packages, in the sense that we would have to argue our way through many layers of admins of different systems, to incentivise us to get patches accepted upstream.
When you have time, could you write up the details of the situation on your end? Something similar to the wiki page I posted earlier - or you could also just edit that directly, if you wish.