So for container trains at least, a programme of loops and sidings lengthening would be the more pressing requirement, before actually and absolutely having to upgrade the couplers, but that kind of infrastructure modification doesn't come cheap or necessarily easy, either.
Another difference to consider is that in Europe, passenger traffic is much more widespread and common than on North American railways, and also commonly has priority over freight trains.
While in principle, longer trains are indeed more efficient than an equivalent larger number of shorter trains, past a certain point they'll eventually become too unwieldy to fit inside the schedule demands of passenger traffic. There's certainly scope for moving beyond the current 750 m maximum, but the multi-kilometre range would probably be pushing things a little too far without completely dedicated infrastructure.
The old-style couplers used in Europe are too weak for longer trains. "The buffers and chain coupling system has a maximum load much less than that of the Janney coupler. They allow around 3,000-4,000 (metric) tonnes total train weight depending on the how they are constructed. The Janney coupler sometimes is built for 32,000 tonnes (31,000 long tons; 35,000 short tons)."
The EU attempt to standardize on an automatic freight coupler across the entire EU was a flop. There's a new project under way to test a new generation of freight coupler, one that connects the mechanical, air, and electrical connections.[1] But it's just getting started.
[1] https://uic.org/com/enews/article/germany-rail-freight-trans...