Yes, the cleanliness of French stations isn’t that great but it’s not worse that London on Friday nights. What’s unique though is that they made all terminii stations into through stations: Gare du Nord, Gare de Lyon, etc….

In London, all trains stop in St Pancras, Waterloo, etc… Not sure why they could not resurface in Liverpool street from Waterlook for instance, the tunnels are almost all there (the W&C line, though they’re probably too narrow but could be extended to Moorgate and thereafter).

Yes, Châtelet smells and is overcrowded, but so too are loads of underground systems all over the world. The Paris RER at least has the merit that it’s high capacity and (relatively) fast and efficient.