In general, it isn't really in dispute! And not just here, but also in real life: IME, it is common to find that all the UD seats (and not just those available for payment) have been taken long before there is any real pressure on main deck space.

I don't think that this change was very recent, but when BA introduced the facility for pre-allocation for payment for those who previously could not pre-allocate at all, the UD was originally excluded. When the change came and it was first discovered that some UD seats could be paid for, there was some consternation here. But things have settled down.

Although BA is not alone in protecting some seats for higher-tier frequent flyers, I think it's fair to say that it's stricter than most (all?) similar airlines in doing so. Part of that approach is seen in the very concept that you can only get pre-allocation for payment in business class if you're not a higher-tier member - a policy which routinely causes apoplexy amongst many here who don't like paying. (Of course, many of the serial complainers still do pay, and do so repeatedly.) But those who benefit from the policy, and who get a wider choice of available seats even if they book relatively late in the day, generally tend to like it.