The current list price for a battery pack fitted to a Nissan Leaf is less than the replacement cost of a new diesel engine! That is supply & fit.
The reality is Petrol engines last 100k miles & cost £3.5k to rebuild, and diesels last 150k miles and cost around £5k to rebuild on a normal car. Of course by then, a new gearbox is required too, with clutch, dual mass flywheel etc. Long before those mileages, the engines have lost a lot of power & efficiency which is why railway engines are overhauled so often! I know that partial top / bottom end rebuilds are much cheaper, but that only provides a short life extension.
EV batteries are lasting 400k miles with only 10-20% losses, and you only need to replace cells to the whole pack if there is a problem. Keeping the charge in the 20-80% range extends the battery life a lot. No power or performance is lost in that time.
Hybrids have tiny battery packs the go from full to empty at great frequencies and being that an old plug in Prius could only do <10 miles on battery anyway, that is no loss.
With hybrid buses, the tech ages quickly so the secondary markets have little interest in spending money on a bus on a school contract or low return bus route. A bus is probably only second to a Uber Prius for heavy battery abuse cycle wise