They said in episode one that you need Benamite crystals for slipstream which are rarer. I still think this whole Dilithium thing is silly. They should be long away from that still being used.
'Timeless' episode also established that they can synthesize Benamite crystals too... but that the process would take years and they degrade relatively fast (both are limitations which SF/Federation would have likely been able to surpass and improve upon in the 10 to 20 years after Voyager returned - along with the phase variance issue).
I agree though that 930 years in the future the Federation should have been WAY past use of dilithium and M/AM... probably should have reinvented its own Spore Drive by then.
I wouldn't object to dilithium being critical for all conventional warp, regardless of the power source. That is, you always have to pump that power through a dilithium crystal if you want it to become warp power: it doesn't matter whether the power comes from m/am annihilation, fusion, a spare Kelvan outboard engine, the burning of coal or the frantic pedaling of 4.7*10^18 hamsters, it needs the dilithium as a filter anyway.
We know that there is magic involved in warp drive at some point: it's fundamentally unreal. We can decide where to draw the line, but drawing it at lumps vertenium cortenide going FTL when energized with real-world power isn't the best way to go. Warp plasma itself is already magical to a degree, as in "Fair Trade"; not just any old plasma will do there. And dilithium is a magic material by its very nature, while m/am annihilation is a known thing in reality, so drawing the line right there should work fine.
We have never had a pressing reason to think Romulans could manage without dilithium (which they mine from Remus, say), and now we have a pretty pressing storytelling reason to think that they couldn't. This should tip the scales on the otherwise ambiguous AQS power issue nicely enough.
Timo Saloniemi
Warp drive isn't really 'unreal' and real life theoretical models have been proposed which scientists are grabbing a serious look at. Its just the energy requirements are seen as too big right now.
At first, researchers postulated you'd need an exotic power source (such as say antimatter) the size of the universe... which they later reduced to size of Saturn (which is still humongous, but microscopically tiny compared to the size of the universe... give them a bit more time and they will likely further be able to refine the theory and reduce the power source size down to something like a football).
While Romulans still use dilithium yes, it was never established that the Quantum Singularity core needs dilithium per say.
Its possible the technology was incorporated into most of their Warbirds in the late 24th century, but I doubt ALL Romulan ships would have quantum singularity cores... and if they trade with other species or use dilithium for something else (as I imagine it has other uses), then it would make sense they still mine dilithium.
Its also possible that the Romulans were crippled by the Supernova which may have destroyed the information on how to build new quantum singularity cores... but this makes little sense because each and every ship comes with its own set of data to help chief engineers understand how the core works, to affect repairs, etc.
Or, its possible the Tal Shiar may have suppressed the information intentionally after the Supernova to keep the technology for themselves.
We also don't know what kind of power core the Valdore warbird (the one from Star Trek Nemesis) used.
Its possible when Shinzon grabbed power, it destabilized the Romulan Star Empire and we've seen the Scimitar did manage to reign supreme over most ships... so its possible Shinzon simply crippled most Romulan warbirds before appointing himself Preator and just used dilithium and M/AM for the scimitar (Remus certainly had enough of dilithium).