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Much less than half as it avoids the need for continuous rails.

There are rail-less bus systems running on pantographs.

I guess that's still less than half though.

Everything old is new again: https://en.wikipedia.org/wiki/Trolleybus

London once had the world's largest system: https://en.wikipedia.org/wiki/Trolleybuses_in_London

Hungary, Budapest has been running battery augment trolleybuses since 2005. They are super handy if you need to divert them for any reason and also it allows for more flexible routing but their range is quite limited, only 4km.

They also experimented with a few electric buses and that also had range problems but much more importantly they realized if they had hundreds of buses charging overnight it would be a significant strain on the electricity system. Distributing the buses throughout the system and their charging in time by charging them at stops would be a solution to this.

With a ton more flexibility, sure.