This may be the case, but it's the start of the rot. If it can't be tested properly, how long before it's deemed unsafe to occupy the road with other vehicles on a full-time basis?
I think that's kinda the point of the new MOT rules though, if it can't be tested properly, rather than banning perfectly usuable cars, just let them carry on whilst being fairly confident that 99.9% of 40+ year old classic will be in good condition. Remember, removing the MOT requirement DOES NOT remove the requirement for a car to be road worthy. As you said, it's fairly easy for anyone to judge if a classic is roadworthy, why can that person not be the owner, rather than an MOT tester? Only 0.15% of vehicles registered for road use are 40+ year old classics - it's really not a significant number to be concerned about.
Within a ten-mile radius of me there are at least four garages that are very familiar with classics and capable of testing them, and while younger testers may not be familiar with them I don't think that's a reason to cease testing them. They should be trained. Hell, in most cases an issue with a classic will likely be a basic mechanical problem. Are they not trained to recognise those in the first place?
I get your point and yes, they should be trained, but these decisions are based on cost versus reward. It's like compulsory insurance for cyclists - it would cost a lot of money to resolve a problem that doesn't really exist and the rewards would not me commensurate with the cost.
Sure, some braking systems and steering play may give them grief - but it shouldn't be hard to work out what's tolerable or not.
That's another item they tried failing my Westfield on - said both front wheel bearings had play in them and it was a failure item. They are designed to have a little play in them. I had to dig out a Haynes manual from 1987 to prove this too them!