I have grandfather rights as does Mr EM so we didn't have to bother with tests for either our 7.5t lorry or trailers, however, our son had to do his Cat C and then as he had that did his C1+E which were fairly expensive. To be fair he was pretty competent anyway as he had started driving our lorry with us supervising before they introduced the rule that those with grandfather rights couldn't supervise.
However, I do totally agree with those who say some people with grandfather rights could really do with at least doing some training if not a test, as someone else said, the number of Pony Club mums who have a meltdown if they have to reverse is ridiculous, I have often found myself reversing trailers for them. I also clearly remember having to reverse a caravan downhill round a hairpin bend (Grogport, Mull of Kintyre for anyone who knows it) because it had met the milk tanker half way up and neither of the couple in the car could reverse! In general, the younger people I have seen who have done the test do seem fairly competent so I think it's a good thing.
The worrying thing I found is that some people don't realise they have to do a test. We sold a lorry and I asked the lady buying it if she had the correct license, she assured me she did as she'd "passed her test a million years ago", a few months later the lorry was for sale as she had passed her test a few months too late to have grandfather rights. Two of my liveries in their 30s are also looking at getting trailers and seemed very surprised when I said they would have to do a test, they will be very glad if the law changes!
One other point is there will be a few businesses badly affected if the law changes, currently those training B+E up here have long waiting lists, partly due to high demand for test spaces following covid, their business could disappear overnight.