I think unfortunately the answer is in your question - from BA's perspective, business travellers should pay for flexibility - whether in the form of buying a flexible Club ticket, or losing any upgrade benefits when they need to make a change.

I'm afraid this one comes down to "the rules". It's BA's scheme, and they've published a framework under which the upgrade option is available. I'd imagine when MFU's came into being back in 2003, there was probably much gnashing of teeth from Revenue Management about the prospect of business travellers who would ordinarily have bought a flexible Club ticket (£££) instead buying a flexible WT+ ticket (for ££), upgrading and then getting the same benefits. Which probably lead to the rule that tickets can't be changed after departure coming into being.