Those are still single vendor systems so if your vendor disappears - as has happened - you are stuck. You also don't get competition for spare parts so who knows if the price will be reasonable in the future.
Isn't it pretty similar for any regular railway? Sure the physical track might be more standard, but that's not where the expense comes in, it's the signalling system, the trains, and especially the interface between the two - and that's all going to be the same either way.
It's arguable for modern rapid transit signalling systems, because those tend to be a bit more bespoke than mainline railway systems and you might not be able to switch to a different supplier without swapping out all the interlockings, and the trackside and train-borne equipment in one go, but for the trains I don't follow your point – switching manufacturers when ordering the next generation of trains is par for the course, and having the signalling system of your choice plugged into the train of your choice is standard procedure even on rapid transit and light rail systems.