The higher cost here is borne by the consumer though, not the advertiser. Javascript shims to 'play' videos do not cost the advertiser anything, just the user's electricity and (ultimately) society's energy usage.

The consumer is already bearing an unreasonable amount of cost to support advertising. The philosophy here needs to shift from "You kicked over my sandcastle, what can we do to make you not want to kick over my sandcastle again" to "Advertisers are a parasite and need to be contained in a separate sandbox and never let out"

The idea that advertisers are the only thing enabling a free web is bogus and needs to be dispelled, the costs of hosting a small amount of personal content are negligible and prior to the advertising era the occasional dude with cash would just altruistically host random niche websites on a subdomain.

Can you explain how servers and bandwidth that handle ads on ad network side would be unaffected by serving bigger loads?

If we are talking about society energy usage it would be ideal to ban ads/adtech entirely.

In the world of S3, cheap CDNs etc the cost difference would be negligible. The user difference would be more pronounced, though, as their limited bandwidth (particularly on mobile) is taken up downloading a bloated GIF rather than something more useful.