Bahaha, what's this full time Exchange admin you speak of? It's not 1998 and we're not struggling to keep a Exchange 5,5 box running on dedicated hardware anymore.
There is no Exchange admin anymore, at least not at any company under 1000 users or with fewer than a couple of servers. That work is done by the same admin team that manages AD, file sharing, etc, and is mostly part time.
If Exchange was your sample company's only server, then I totally agree O365 is ideal. But in most medium sized companies Exchange server isn't even a drop in the bucket anymore unless you're doing something really stupid with journaling. In the era of virtualization, the data center space, power, hardware, and nearly all the expertise is already purchased.
The marginal cost to run Exchange is trivial if you already have this infrastructure in place. The O365 math is based on these false ideas about "dedicated admins" and a bunch of dedicated hardware that went away years ago. In any organization not run by retards, running Exchange competently shouldn't be a major burden.
If you're running enough mailboxes/servers for Exchange that you can justify a dedicated admin (which I assume would be dozens of servers, many DAGs, a real complex mess) I'm not sure if O365 is a "fix" at that point, either, because now you're talking such a large userbase that the O365 licensing gets into real serious money.