[lxgr]

For one thing, they can't show ads in third-party clients. (I think they currently only do that in "stories" in their own client at the moment, but without competing clients, they always have the option to expand that.)

On the other hand, I could imagine their business messaging ambitions to be threatened by third-party clients: There's nothing stopping the vendors of these clients from undercutting Meta on business messaging rates.

And this is all assuming that third-party services couldn't provide their own business messaging services to first-party WhatsApp client users without paying Meta their list price. I don't know whether the DMA allows that, or if normal rates would still apply in that case.