Blender would not rely on it. If issues occurred over time due to the proprietary nature of the platform, they could just switch to something else.
Ever heard of vendor lock-in?
If there is an open source [sic] solution fulfilling the requirements, even better, but that doesn’t seem to be the case.
On the contrary, SourceHut is the fastest, (anecdotally) most stable, most usable (working efficiently via email) and most freedom-respecting and ethical option at the moment, aside from Savane (which I wouldn’t propose using because it’s technically inferior to SourceHut), scoring a B on the GNU ERC.
(The issues it has which I previously mentioned in other posts can be fixed relatively easily.)
They are currently using an open source [sic] solution which was abandoned. So there is clearly a risk when relying on open source [sic] too.
There is always a risk, it’s software written by humans who make mistakes after all! Imagine if the current solution were proprietary, the Blender project would have no choice but to switch away from it, but since it is free, they had the option available to fork and maintain it, which is an advantage over if it were proprietary.