They are not shifting for everybody at the same time, they are shifting only for “tech companies”, who are currently perceived (rightly or wrongly) as undercutting “normal businesses” in an unfair way. (Scare quotes because in practice I don’t trust the UK government know what they mean by the words).

Well, then tech companies will raise prices and product makers will buy fewer ads, leading them to increase the ad load, or similar. It demands on the demand elasticity.