I think once you decide on a McLaren and then move within the different products, you recognize the individual differences, they are certainly there, in that sense I agree with you.
But I dare say that the prices of used McLaren's (this also applies to the LT) are primarily defined by brand newcomers. So it seems important to me regarding the question of what is happening with the 675LT price right now, how does a "non McLaren customer" view the product situation? And with such a prospective customer it is much more difficult to convey real differences between all the models.
A newcomer to the brand will first notice that all McLaren models are two-seater mid-rear engine cars with a (more or less the same) V8 bi-turbo engine and carbon monocoque. Even a GT works exactly in this pattern at McLaren.
With Porsche there are already big differences in the choice of engines, turbo or naturally aspirated, then mid-engine or (mid)/rear engine, manual transmission or dual clutch, etc.. With Ferrari, the product differences are even bigger: V8 or V12, 2+2 seater or two seater, front-mid engine or mid-rear engine, etc.
From this point of view, if you summarize all its products under "two-seater mid-rear engine concept with V8 turbo drive", McLaren is almost a mass producer.
For a McLaren owner the question arises immediately, why should I pay so much for an "old" P11 LT when I can buy a new LT for a little more money (e.g. 675LT vs. 600LT) which offers me exactly the same racetrack-oriented setup and is able to deliver at least the same performances? With such a prospect profile the only question that makes sense is the speculation on value increase. But this you can forget in the case of the 675LT with its 1000 "limited" vehicles, because this amount of produced cars is almost a third of all 650s produced.
Of course this is a thread for 675LT owners and they defend their choice. But in terms of value appreciation, it looks bad. Which does not mean that the car is bad! However, even if it's a superior car compared to the brand-different competition, in the end, the 675LT is simply a racetrack-oriented McLaren, as there are various others within all these McLaren products. By the way, this car is still my preferred McLaren. But my personal preference will unfortunately not drive up the price of the 675LT.