It's a somewhat low hanging fruit. And every little bit counts

We should not make perfect the enemy of good

Diesel buses pollute badly. Both emissions and noise.

They are just better vehicles, both in terms of local pollution, and in terms of propulsion, even if it is green washing.

As long as they can last all day then charge enough overnight I think they are a big improvement. Air and noise pollution sucks with diesel.

My fear is in extreme cold places, where EVs tend to have issues.

I'm not sure I follow your logic. The environmental impacts of manufacturing will be averaged out faster with something that's getting a lot of use, and targeting the efficiency of something that gets continuous use has more impact.

Throwing away busses to switch would be silly, but it's a perfectly good idea to switch as you maintain your fleet. Your alternative is buying more diesel busses, which... No, building a trolley or tram system which is more expensive and less flexible, or rebuilding cities to be livable without needing vehicle transportation, which isn't happening soon and has trouble for people with mobility issues like the elderly without other some accomodation.

Like a lot of things, this case has multiple sides and is not as simple as calling it green washing.

On one side, a packed diesel buses is far more efficient and probably pollutes less per person than an electric car with a single occupant. So in a way, it's indeed putting efforts on something that's not very high on the list of things that pollute.

On another side, diesel buses are still polluting and the local transit authority here has a multi decade plan to electrify them. It has its own studies, and studies from the nearby universities, showing that electric buses can be attractive and effective for different types of routes, and less for others. They are replacing the buses according to this plan and it's nothing to be against. I don't consider this green washing, coming from a transit society, but it's an easy target.

But yeah, electric buses from your transit society is a nice plus. However diesel buses are also just fine, and even desirable, if you use them as an alternative to driving and to reduce pollution from car traffic.

This reminds me of an old meme that I kept.

In some places that had trams, electric buses will connect to the tram power lines and charge up a bit (or at least use less of their stored energy) while in those areas.