The end users buy the products of the advertisers. In a monopoly ads cost more and to make a profit advertisers have to increase their product prices. In turn the users pay higher prices for the products that are being advertised.
TL;DR Businesses just pass these costs on to their consumers.
Businesses have somewhat standard target gross margins, customer acquisition costs (CAC), LTV:CAC ratio, etc. and will determine pricing based on multiples of these variables. Ad spend is often a huge component of CAC and this directly feeds into what companies need to charge their customers in order to use their products. The assumption that businesses will just eat a higher cost as sort of consumer benefactors is quite naive. I can assure you they won’t.
So monopolistic practices will harm the consumer (advertisers), but do they really harm end-users? Does a monopoly in farming harm the cows?