The SepratistsSeparatists were standing against Thethe Republic, and yes, they had a huge droid army. And, of course, the people responsible for procuring that army said it would wipe all the foes from the table. Who DOESN'T tell their customers/government that their solution solves all problems?
As such, the SepratistsSeparatists had a Plan B (which just so happened to be right up Palpatine's alley), the Death Star. It would take much more time to build, and take many more resources than the droid army, but it would ALSO serve as a blockbuster weapon to break any stalemate they might have experienced in the future.
There were likely many skunkworks projects being developed by the sepratistsSeparatists at this time, from Death Stars to toxic bantha fodder. ALl ALL the ideas were on the table, and the more promising ideas were promoted to the top, even the far-fetched ones. Win, lose, or draw, a Death Star would be a handy thing to have, if it could be built.
LEGENDS: SuperweaponsSuperweapons were just a thing people did in that galaxy far, far away.
The Star Wars galaxy is FULL of superweapons, starting with Centerpoint Station and going on through multiple Death Stars, World Devistators, superlasers mounted on ships, etc., etc., etc. The Sith Empire had superweapons. The descendentsancestors of our heroes centuries+ BBY had super weaponssuperweapons. The Tarkin doctrine wasn't so much a new plan that needed superweapons so much as it was an explanation on how a superweapon could be used to enforce fear.
The US Army was pretty confident that it could storm Japan; itit was called Operation Downfall. Operation Olympic would capture the south third of Kyushu, using Okinawa as a stage. Six months later, Operation Coronet was planned to invade Kanto.
Of course, the problem was, that Japanese geography made planning the invasion simple; therethere was only really one good way to do it. And the Japanese knew it too. The Japanese empire planned an alloutall-out defense of their main island. And as Japan slowly lost control over the Pacific, they were drawing back planning for that ultimate defense.
Yes, even though the SepratistSeparatist government used droids, not living beings, those droids do have a non-zero cost. It takes time and effort to build them, transport them, program them; anyany "conventional" operation would include massive losses on both sides.
But both these situations had a superweapon: onon Earth, it was the Atom Bomb.