In every practical sense you are correct. Anything you could do with a battlemech you could do with conventional armor or aircraft for cheaper. About the only thing you can get away with is that with the neurohelmet interface you can get away crewing a machine with that level of firepower with a single crewman rather then a team you'd need something from modern day, and even that's a stretch since Battletech has the same equipment used in some tanks, and some mechs make room for a gunner seat.
The Pre Game history of the Age of War DID have full scale war using conventional weapons, complete with nuclear weapons where most arguments were won with warships and orbital supremacy. The abuse of both made warcrimes the norm to a point where the powers that be had enough and passed the
Ares Conventions which dramatically restricted the ability to use those tools in the theatre of war moving forwards. At least until the Succession Wars kicked off, and folks nuked themselves back to the iron age for a few centuries, and got to the point where they couldn't actually fight like that any more because every time someone found or build such things they got wiped off the face of the planet.
But between the Conventions and the loss of tech, war downscaled to a point that you didn't fight to destroy, you fought to take and hold ground and resources. That slowed the losses of civilian life and habitable worlds, but also shifted funding and resources to tactical rather then strategic weapons. In this restricted form of warfare, the idea was now 'doing more with less'.
When the first battlemech, the Mackie, came out of what ever wild fever dream they had in the Terran Hegemony for maintaining their technological edge over the other powers in the galaxy, it's first demo was taking out a quartet of conventional battle tanks. That demo scared the piss out of all the Houses, and suddenly everyone was rushing out to steal the plans and make their own versions.
Then you came to the cultural side....mechwarriors were a new and crazy breed to be running these things, and they became the new Knights Errant of the feudal culture of the Innersphere, including a code of conduct, which again goes to the 'restricted' form of combat.
While the IS like to pick on Clanners for their Honour clouding their ability to fight an actual war, they're honestly nearly as bad, because using mechs is a point of Pride rather then effective warfighting.
Then the Word of Blake came in and started popping off Nukes again and we get right back to where we started.
