I was thinking more like the drone zephyr but you make a good point about the 'Mechs. I could see a ground based command station in a fortress controlling a group of drone Behemoth tanks for example.
If you put some value on either the lives of the crewmen or what you have to their training and salary, then the argument for drone tanks is actually easier than for drone mechs because most tanks require more than one crewman. Assuming one drone crewman remotely controls one drone tank, you free up crewmen and can field more tanks with fewer crewmen using drones. Because they are crewed by only one mechwarrior, mechs don't benefit from this argument. But most combat vehicles do.
Like the Lich, the omni-Zephyr carries Angel ECM, so it has the added benefit of being hard to disrupt with other ECM systems. Although Angel ECM is expensive, if I was investing in high-end tanks like Behemoths, I'd probably add Angel ECM so I wouldn't lose control of them to the first Succession Wars-era Raven that strolls by.
Lich snipers in the rear screened by Zephyr As and Bs up front make for a decent all-drone force. It's too bad the Lich doesn't carry C3 to benefit from the Zephyr B's targeting data.
Edit: What prompted the question was the Scorpion Tank article in Fan Articles. Several people mentioned how terrible it is and how you're dooming the crew to a horrible death wen I thought "What if it was like a drone? Put a Drone OS in it and go to town."
At some point, the costs of drone systems overwhelm the economic benefits of cheap tanks like the Scorpion. Cheapo tanks have to operate in numbers to be effective. Can I still buy enough Scorpions to meet my needs if I'm adding drone systems to them or am I better off buying more Scorpions and crews? This becomes an especially difficult economic argument if we're also adding the costs of ECM (especially Angel ECM) to the cheapo tanks to protect their operability as drones.
To get the biggest defensive bang for a given number of crewmen, I could definitely see the Republic converting or fielding large drone vehicle garrisons. Would be a nasty surprise when the walls come down. Same could have held true for the Blakies in the closing stages of the Jihad or the Society in the Clan Homeworlds.
FWIW...