The North London Line is useful because orbital bus routes are something of a novelty in London. Most of the hundreds of bus routes radiate into Central London or serve to link residental areas to the nearest pair of town centres/railheads. Only around a dozen or so operate as lengthy trunk orbital services. There are even less routes that parallel railway lines during the day. One exception is route 345 which near parallels the SLL between Clapham Junction and Denmark Hill and unsurprisingly is a 24 hour route. The NLL isn't well covered by parallel bus services.
Yes, you can get almost anywhere in London with ease at night, but options are limited. If you want to get from Stratford to Camden or Highbury when the NLL is finished, you have to take two buses and a long diversion via Central London.
The other problem is that when capacity increases, the trains just fill up! Have a look at the Wimbledon to Waterloo corridor. The dynamics of that line are interesting, it is virtually physically impossible for the trains to run on time in the morning peak as the trains are rammed. They can't officially amend the schedules to account for the ever increasing dwell times - even an extra 10 seconds at each stop might cause a breach of the ROTP headways, necessitating removal of some services, which in turn might not be compliant with the franchise service level commitment. So the trains run late every single morning, but have quite a bit of slack at Vauxhall.
I even ran a perfunctory Monte Carlo Simulation based on LDB data for all 50 arrivals between 07:00 and 10:00 and found that a commuter travelling on routes involving that stretch of line should expect to be delayed on average by 3 minutes and 48 seconds into Waterloo

. Everything they've tried is being trumped by suppressed demand - refurbishing the 455s, improving flow by removing seats and widening the vestibule and increasing the frequency of service. They're introducing 10 carriage trains, but it won't take long for those to fill up either! The only thing that hasn't been tried is improving the time it takes for the doors to be released - sometimes the train rolls in and it takes 10 seconds for this to happen, whereas it's more instantaneous on DOO-P services.