I'd speculate one problem with 'improving' DLM would be extra complexity. And it is nearly always a mistake to try to fix a flawed algorithm by making it more complicated.
I'd also suggest that BT get an actual measurable advantage for themselves and their customers by making DLM as aggressive as possible, as long as it doesn't interfere with actual real-world usability of customers' broadband. Retransmissions do consume additional network bandwidth which, effectively, is wasted bandwidth, and aggressive DLM would reduce that wastage.
Speaking for myself, my line does appear to be under DLM control as every resync event (I average about 1-2 per week) seems to result in exactly the same speeds as before. Then after a few weeks, the same thing, but with a different speed cap, sometimes higher, sometimes lower. But does that bother me? No, it doesn't, as long as the various services that I use continue to work albeit with variations in elapsed time for uploads/downloads.