At the same time, WPP did have some weaknesses.
It focused on growing its media buying business at GroupM, which may have distracted it from focusing on digital-focused agencies, said Ken Robinson, partner at advertising agency consulting firm Ark Advisors.
Kassan argued that WPP's investment in GroupM and hiring top digital media buyers showed it was thinking toward a digital future, but he noted it did lag behind competitor Publicis at acquiring top digital agencies.
In addition, media agencies have drawn scrutiny in recent years, especially after a 2016 report from advertising trade group ANA showing many groups were receiving kickbacks for spending more money at certain places or taking advantage of rebates without telling clients.
"[Sorrell] is a genius," said Sandy Greenberg, CEO of creative agency Terri & Sandy. "He's an innovator. He's an agent of change. He brought brilliance. Sadly, he's also representative of all the excesses of the humongous holding company model."
Greenberg added, "People are not trusting the agency model because of bloated excess and lack of transparency."
Many brands are moving media buying in-house instead of using agencies.
The world's largest advertiser, P&G, said during its earnings call in January it would take over more of its own advertising purchasing to cut agency costs, according to Ad Age.
Another ANA report from December showed 35 percent of companies moved their programmatic ad buying away from media agencies, according to Adweek. Programmatic refers to an automated method of buying ads on digital properties, such as websites and social media platforms.
"[Sorrell] was representative of the old guard mechanism of advertising, being the agency of record, the methodical way of an agency working with big brands," said Ari Lightman, professor of digital media and marketing at Carnegie Mellon University. "That mode of being the agency of record is kind of gone."
In addition, social media profile data is allowing companies insights into their own customers. Platforms like Facebook and Google allow brands to buy directly from the company, leaving less room for a middleman.
"More [companies] can do the mechanisms of advertising segmentation, engagement, reaching out, activation and analytics," said Lightman. "A lot of [advertising buying] has gone towards programmatic buying. A lot of it is going towards these digital sources to do it on your own. A lot of the power has been stripped away or devolved away from some of these large agencies."
Lightman added, "The notion of a large agency that does everything for a brand around advertising and marketing is going to go away to more niche, more nimble folks who are more digitally oriented, folks that understand the medium and understand how to acquire data."
To be fair, everyone agreed that advertising and media agencies will always have a place. But as platforms like Facebook and Google give companies unprecedented access to consumer behavior, they may be less important than they were before.
"There are still good reasons for their existence, including their talent and capabilities," Kassan said. "The onus will be on the holding companies to reinvent their legacy structures and become the modern, agile and responsive partners that marketers and consumers demand."