3rd rail - outside of underground metro operations - is a technology which was at best substandard and at worst obsolete 70 years ago. Only the UK undertook widespread electrification using this inefficient and frankly dangerous standard.
Over time the mainland 3rd rail mainlines will, quite rightly, be updated to 25kv OHLE. The Island Line will never justify this, but will easily justify battery or hydrogen. The move to battery could, and probably should, have been done now, proving the technology and improving safety.
It is, of course, a historical accident that the Southern ended up a third rail system. They could have chosen the LBSCR low-frequency, high-voltage AC system, but I can understand why it was dropped. By the time of the Grouping, there was much more third rail mileage, and the LBSCR system was expensive, dependent on foreign components that had been unavailable during WW1, and technically rather complicated, essentially being some years ahead of its time. Also, apparently, the motors on the trains needed a very high starting voltage, and the resulting jolt when they pulled away from stations was enough to give you whiplash.
Given that the Southern wanted to electrify quickly and cheaply, they made the right decision at the time, but the system then became a victim of its own success. It became one of the biggest electric networks in the world, and changing or mixing systems would have become incredibly difficult and expensive to do.
The Southern considered 25kV for the Bournemouth line, but baulked when they realised how complex and expensive the knitting would have been going into Waterloo. Remember as well this was a good decade before BR's first-ever dual voltage trains appeared.
I agree third rail is obsolete and needs to go, but unfortunately getting rid of it is a huge job! Island Line was probably perfect for battery trains, and I'm not convinced the decision to keep the third rail was a great idea, but I guess the ship has sailed on that one. De-electrification is a bad look, I suppose.