BT is a game with its lore, and there can be worse weapons than others.
And that's the thing. BT does have lore. If this were back at the beginning, and the plan was to make Autocannons a technologically competitive weapon versus amplified light- and particle beams, then, sure. But, it seams that there has been a limit to explosively propelled shells, compared to magnetically propelled projectiles. There's only so much powder you can put behind a bullet before the barrel weighs so much in order to contain and funnel the blast.
So, the AC, while at one point in history was still a super weapon (versus primitive armors), it has seen a cap on its capacity in the form of LB-x and Ultra, and questionably Rotary, as well as alternate munition types.
Think of what the setting was as described for the second ed box. The Succession Wars saw a lot of manufacturing slowed down, and Mech replacement weapons kind of dwindle to salvage. Guess what was around to salvage? Old autocannons!
With a construction ruleset for the game system, it becomes obvious that some weapons should be replaced, whether that was intentional from the start or not. It gives the universe some flavor.
The only counter to that is that most of the AC/5 toting designs seem to have been designed that way from the outset, with the AC/5 being a defining feature of the variant which 'sold it to the Quartermaster Corps. There probably should have been 'more advanced' versions that quickly replaced the AC variant during the height of technology, before the old specs were brought back out.
The Jag is an exception because it was designed after the fall of the Star League, and by the Davions which have a personal love for Dakka.
So, if they re-worked the armor in the video game, did they play around with the tonnage for ACs? That, if anything, would be a good fix.