I can't see any TOC going, "we're not going to bother ordering now in case someone else chooses the same supplier and delays things".
I was thinking from different perspective, might have got indicative build timeline from a manufacturer months ago. But if that manufacturer gets a big order from elsewhere in meantime, then those dates would slip.
We already know hundreds of EMU and BMU carriages have been ordered in 2026 by European operators. No manufacturers are going to build a completed train if important components are delayed because the parts supplier has orders for next 2, 3, 4 years.
It is a limitation of buying in parts (rather than making in house) could be bearings, door components, wiring fittings or whatever. Imagine a scenario where get something like can't fit toilet roof tank, because toilet module needs to be inserted first, but they are delayed months. Whole process logjams.
The other point is that if planning to order replacement trains, it determines when stop expensive heavy overhauls, or life extension work on existing fleet. Get timeline wrong and incur extra costs of patching up current fleet, or incurring penalties on lease return, or extra expensive short term lease extensions where backed into a corner if need bridging period until new fleet ready