You just added two expensive locks and a shed to the cost of a bike
Let's say you've just brought a £500 bike, and you need a new shed; well my last shed (7 foot by 9 foot,) cost £1,200 (earlier this year) installed and with the old shed taken away.
So far compared to the cost of car ownership you've mostly covered 12 to 18 months if fuel and insurance.
2 decent locks at £50 each and that's VED and maybe an MOT for a year (depending on the duty owed on your car).
As for backpack/panniers with a rack and lights that's only a service for a car.
We've not even looked at other costs (like buying the car, parking, new tyres and other parks) and we could still do all that every year!
In reality my last shed lasted 12 years before it needed replacing (mostly due to having no children when first installed and there's now a few kids bike and a tag-along), whilst at the opposite end my lights are fairly new they're still a few years old. My lights now are USB charging, so no need to keep buying batteries which given how fast your can get through them otherwise is worth the higher upfront costs.
As for my bike its over 15 years old and whilst I've had to replace tyres, breaks, chain, rear set and a few other parts it's still going strong and still a half decent bike.
I've probably spent less than £50/year on my bike, parts and accessories, and even with the sheds (the new one is less than a year old) and it's about £125 - so less than the "standard" VED cost of £140.