Airlines do not usually mix Domestic and International passengers on the same plane
This is because unlike trains, planes don't generally pick up and set down passengers en route (except on the longest routes). For trains, it is an efficient way of serving multiple communities with the same service. For planes, the cost of multiple landings (both in time and resources) would make this wasteful. The imposition of airport-like security theatre on cross-Channel trains (yes, I understand it's not exactly the same) forces these to be more like planes. There can be no doubt that preventing cross-Channel trains from carrying domestic UK passengers is a major factor in so few of them stopping at Ashford; it is also why there are no trains between the north and west of the UK and mainland Europe (when such trains were originally planned).
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That's very interesting, conclusively demonstrating that pre-boarding inspections are more practical and economic than British officials performing the operation on the train.
Perhaps, but train services across controlled borders elsewhere generally have border checks performed on the train (or at the border station). It's not clear why it should be possible there but not here.
There might be an argument that such restrictions would be unacceptable within Shengen, but these would be additional trains on routes which other services ply so it should be possible to choose to wait for a later mainland only non controlled train or catch the earlier controlled one.
On the main route where this is an issue, Lille–Brussels, about half the through trains are Eurostars, the rest TGV. These are timed to complement each other (to the extent that the timings are organised at all). If you need to travel to from Lille to Brussels in the evening, for example, you have to take a Eurostar: the last TGV departs Lille at 17:07. [You can, of course, take classic trains, which take at least an hour longer (over a 35-minute journey by fast train) and involve at least one change.] The Eurostars are an integral part of the high-speed service between Lille and Brussels, so making it difficult for passengers to use them is not desirable.
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Do you have any figures to prove there is a gradual reduction of international train services all over the EU? My perception is that the opposite is the case, with TGVs running to Germany and the recent development of the OeBB Railjet network to take just 2 examples.
The general trend seems to be a reduction of local/classic cross-border trains as high-speed services increase. This was seen most recently in the withdrawal of the classic through service between Brussels and Amsterdam after the introduction of a high-speed service with brand-spanking-new trains, which was then abandoned as these trains didn't work properly, but the classic service has not been restored.
The reason for stopping or reducing the classic service seems to be fear that local services would abstract revenue from the high-speed ones. I think this is mistaken: the local and fast trains serve different markets. But it's what happens. Another example is SNCB's barely adequate 2-hourly service between on the classic route between Liège and Aachen, but SNCB didn't want to run any at all lest it abstracted revenue from Thalys.
Even worse is the lack of any rail service at all between Erquelinnes (Be) and Jeumont (Fr), 2km away on a rail line that runs across the border.
But there are others,like Malmö–Copenhagen and Strasbourg–Kehl, which are models of how a cross-border passenger train service should work. The EU ban on subsidising cross-border services is certainly an issue, and it would make sense to allow governments to subsidise local train services to the first station across the border on the same basis as domestic services. But perhaps another reason for mixed provision of cross-border services is that national operators don't always play nicely with each other.
