For a reasonably sized transport association it will be outright impossible to electrify an entire fleet at once:

1. Where do you get all the buses? For Hamburg alone, you'd need to buy 2600 new vehicles, which would be half of all buses sold in all of Germany in a year.
2. Where do you park those 2600 new vehicles until you've sold the old ones? For some transition period you'd have twice the number of vehicles while at the same time...
3. You'd need to overhaul all your bus depots at once, which adds to the parking problems. In Hamburg we have more than a dozen bus depots, where do you find all the construction workers to upgrade them all at the same time? Where do you get all the architects, planners etc?
4. You'd need to get a lot of electricity to your depots in a short period of time. Your local power company might be able to build a new substation for one depot a year, but 10? Probably not.
5. Where do you educate hundreds of mechanics on the new technology? And who's maintaining your buses while all your mechanics are at the training?

You don't need too much infrastructure to start transitioning: You can add charging infrastructure to on one or two terminal stops, upgrade one bus depot, educate 10% of your mechanics and start by upgrading all the lines going to these terminal stop. In the next year you upgrade the next terminals, the next depot and train another 10% of your mechanics. After a decade you're fully electric without a big hassle.