As was proven last week, Waterloo is a single point of failure. If that stretch between Clapham Junction and Waterloo falls over, that's the majority of the South West out of action until the end of service.
As it happens, I don't imagine that many people on the WLL that travel Southbound are going to double-back towards Waterloo when there are other routes into the city from West London, e.g the District or Central line. The exception possibly being Imperial Wharf.
With that all in mind, we could probably do ourselves a favour by not trying to send more services to Waterloo.
I believe that the WLL could be used in part to provide additional services, but not for Waterloo. Instead, I would rather see a cross-country service that links the Kent coast up with the North. Such services existed 30 years ago and can provide some very useful connections to other parts of the country without having to faff around with a dozen London terminals and the underground. But that's likely a topic for a different thread.