Hopefully it will encourage Debian to continue allowing that choice at a fully supported level.
My concern is that if the people that care about running without systemd all migrate to Devuan, and if Devuan developers put little effort into pushing their changes upstream into Debian, then there will be that much less reason for Debian Developers to maintain the choice.
It takes good bugs, preferably with good patches attached, to keep that sort of functionality viable, especially if the person maintaining the package has no strong views about the init debacle (which is the case for the majority of Debian users and thus developers).
The work that gets done in Debian is that which interests people enough to do it. If those interested in choice of init all go elsewhere then of course that choice will wither on the vine.
If on the other hand Devuan were to act more like a normal Debian Derivative, they'd be making sure that as much of their work as possible was fed back into Debian, they'd be maintaining major components as both Devuan and Debian packages, they'd be reporting bugs where Debian fails to satisfy their preferences, and all that would act to preserve the choice that you worry about losing.