If the manufacturers thought they were being responsible in protecting us, from ourselves and from prosecution, they could have put in place a facility where all of their authorised dealers could take in a rifle (with AT) that had drifted over power (and they do drift over power, some models more frequently than others) and turn it down free of charge. Paid for of course by the factory. Now that would be seen as a comprehensive service and a proper effort to support the customer. It would also support the trade.
But that facility doesn't exist.
AMTA could also have negotiated - or tried to negotiate - an indemnity situation with the Home Office and CPS, whereby any rifle found to be over powered and with the AT intact - and it isn't that hard to make things tamper evident - would not lead to a section 1 prosecution of the possessor. It could be returned to the factory or an authorised dealer for adjustment.
But I'm not aware that such an indemnity was ever sought.
Brand new rifles with AT still get sold today, over the limit. We had a well known sporting rifle at the club not so long ago, running at something over 14 straight out of the box.