There are a number of competing goals when developing software that "the whole world can/will use". And it seems obvious to me that getting the whole world to use a secure, encrypted messenger like Signal (now, Whatsapp) is a noble goal. The obvious competing goal is federation, and the fact that we've failed at improving federated services (e-mail being the best/most important example) is a problem.
Ultimately, Moxie's arguments here hold a lot of sway for me. I think it sucks that constantly-improving services are impossible to address in a federated way, and he's not really offering a solution to that. But instead he presents arguments I hadn't thought about much, which is that perhaps non-federated services can be dramatically better than federated services in the short-term. And in the long term, perhaps we'll move back to federated services; right now, the cost of changing services/ecosystems is very low. So we don't need to be as 'doom-and-gloom' about the fact that Signal does not achieve all the competing goals, because it achieves a number of extremely important goals right now, and we can work on the other goals in the future.