Battery electric buses require batteries, which may be flexible, but are also expensive.
I don't see existing trams and light rail disappearing soon. Not in cities that have well-functioning networks, like Amsterdam. But I can imagine that for new projects, electric buses are preferable.
https://bathtrams.uk/the-most-energy-efficient-mode-of-publi...
Still, I feel that trams will be dying soon. The improved efficiency is simply not enough to justify the extra infrastructure and inflexibility. Modern battery electric buses have a range of 500+km and can thus be used for an entire day, and charged at times when electricity is cheap and green (ie overnight on wind energy).