That's true. We don't know what would have happened. But like Rayla said, the Emperor would have come in eventually.
The way I see it, there are many different ways the war could have gone down.
1. What did happen
2. The Jedi don't join the war at all
3. The Jedi join the war when things get really bad
4. The Jedi join the war right away
Option 1 led to the Jedi Civil War. Option 2 would lead to the Republic being destroyed. Option 3 would lead to the Jedi Civil War and the Republic being destroyed. Option 4 leads to the Jedi Civil War and the Republic being destroyed. But the outcome doesn't matter as the Emperor would have attacked eventually. What matters about these outcomes is who the Emperor would fight.
Option 1 is what we have now in TOR. Option 2 the Emperor takes control from Mandalore, with or without conflict. Option 3 and 4 the Emperor fights the "Sith" and the Star Forge.
EDIT: But these are only a few outcomes and paths that could have been taken.
That's very interesting.
However Option 3 (I think) would have gone better then you say. If the Jedi had joined the war (I mean join mostly as generals) then everything would've been fine. There wouldn't have been an opportunity for people (like Revan) to fall to the darkside. because the moment Revan did something remotely bad, then the Council would have been on top of him and would have prevented any fall.