Welcome to FT.
A lot of people join because they have a specific issue and are looking for a solution or just to vent.
I hope you stick around afterwards.

A lot of posts here say "I will never fly BA again" or similar.
But in reality (as you continue) people won't cut their nose off to spite their face in most cases, so where BA still makes sense for them they will continue to put money in their pockets.
As you have found, BA (and most other airlines) have outsourced their baggage resolution services and have little real control of it, so unfortunately complaining to BA or on here will do nothing to change things.
You have received a lot of advice about the inadvisability of travelling with expensive luggage, advice I would agree with, but obviously that is too late for you now.
You have received other advice that suggests you may not have to accept the offer you have been given and could pursue it by appeal to K2 and BA but would probably have to go to CEDR to get it resolved.
Whether you can be bothered with the hassle of pursuing that is up to you. I know there are some people here who always do it as a matter of principle (and perhaps a hobby) but I value my mental health which would be diminished by this process, so unless it is totally egregious I would never go that far (a common attitude and one that BA seem increasingly to rely on to reduce their liabilities).
I don't think this is a uniquely BA phenomenon.