An engine rebuild is not "servicing" neither is swapping out injectors.No I mean keeping the injection system in good order. Yes injectors need to be replaced from time to time and it's not overly expensive. Again blowby and leaky valve stems can be fixed by a rebore, new pistons, new guides and valves. If you've experience of working with engines I'm surprised you don't know about such things.
People don't rebuild engines or swap out injectors to achieve marginal emissions improvements - the MOT test certainly doesn't require it (the emissions have to be really quite horrendous in order to fail the UK emissions test).
For older vehicles it is pretty much uneconomical anyway (do you know much it costs to rebuild a diesel car engine?). Aging diesel cars are not going to become desirable future classics, (in the electric age they will be a major embarrassment).
Why is your average motorist going to pay to have injectors replaced or pay for engine work when it will not make any noticeable difference to MPG or performance? OK so it will it make it stink a bit less..... - most people don't care ("all that money..... just to make it smell better!") - they are not the ones who have to breathe it. They can just carry on changing the oil and filters and run it up to 3, 4 or even 500,000 odd miles. It will be eyewateringly filthy; toxic as hell, but hey it still does 50mpg - what's not to like?