Speaking to Campaign, Kelly Clark disagreed with analysts' comments that MEC and Maxus had failed to differentiate themselves, leading to the merger of the agencies.
He pointed to MEC being a top five global media company network in Recma's rankings and that it had won numerous media company of the Year awards locally.
Clark said: "MEC has a powerful client list. Clients aren’t stupid. They have been very successful at that. Similarly for Maxus which has a very different proposition and is strong in local markets, such as for BT in the UK or ITC in India. So I would not agree with that at all.
"As Taylor Swift would say, ‘Haters gonna hate,’ but we’ve been very upfront about why we’re doing this with clients."
Clark also insisted that the restructure was firstly about repositioning Essence, the digital agency WPP acquired in 2015, but agreed it was a "benefit" for GroupM generally in terms of giving a group a simpler offer in the market.
"This is about investing in our future for two reasons: One, it gives us the best solution going forward and two, highest growth opportunity for us as a business.
"This is a statement of intent around where we intend to compete very aggressively in a data-driven world. Essence is a real agency doing really great work for real clients—unlike some of the things that have been thrown around by some of our competitors."
Clark hinted that the name of the new agency, which has not yet been decided, may not necessarily start with an ‘m’ in the style of MEC, Maxus or sister GroupM agencies such as MediaCom or Mindshare.
"GroupM has a bigger meaning in the marketplace and I think the industry recognizes that," Clark added.
Clark, who became GroupM chief executive in September last year, would not be drawn on what would happen to London agency chiefs Jason Dormieux (MEC) and Nick Baughan (Maxus).
Nor could Clark say yet where the London agency would be headquartered. Both UK offices have moved last year; MEC to Sea Containers House on the South Bank and Maxus to Finsbury Square in Islington.
However, he could confirm the new agency headquarters will be in New York: "Tim Castree will lead the organization; he is based here in New York and when we have finished the integration of the business the team will be led from there."
Meanwhile, in terms of resolving client conflicts, such as Vodafone (MEC) and BT (Maxus), Clark said that talks with clients had already begun at different times. However, this did not necessarily mean big global clients were informed before local clients.
"We have done quite a lot of thinking around this as part of the planning, but when it happens depends on the client. Conversations have been taking place for some time and it’s not to do with size," he added.