Here in the Netherlands the bus companies are required to buy by tender which has led to the purchase of lots of cheap Byd busses. While they are fast enough and silent they are not that great: there are software problems, heaters don't work, doors won't close and the seats are so bad the (already scarce) drivers are getting back problems.

As a general rule, in transport fleets, unless the old thing is actually basically at the point where it needs to be scrapped right now, the new thing always has more downtime than the old thing, because the old thing has had its kinks ironed out, and the maintainers are used to maintaining it.

Of course, the trouble is, sometimes the new thing is just bad. A commuter/metro train system in my city operates mostly on 40 year old rolling stock. In 2000, some new trainsets were introduced, from two different manufacturers. One model never really worked properly, and was scrapped in 2008; the other survived. The whole thing is now due to be replaced with shiny new trains, starting this year... hopefully these ones work.