First of all, you're looking at things wrong. You're running Exchange and other services on your server as well as Active Directory and DNS. You're doing it wrong. You really want Domain Controllers to only run Active Directory and DNS. You'll run into serious performance issues down the road if you get a medium number of mailboxes in Exchange and it runs on a DC.
That being said, downtime is a real issue. Is your boss OK with users not being able to log in, access file shares, access other SSO technologies that you might leverage for the hours that it will take to do a restore? If you have two DCs (or more) and you have exchange and file services running on separate servers like you should be, then this becomes a very real problem.
As it is, it seems like you already have all of your eggs in one basket, which is a really really bad position to be in. You should be pushing for a dedicated Exchange server, a 2nd DC, and possibly a file/print server. This, of course, depends on the number of users that you have. Even if you do keep Exchange and any file\print services on your existing DC, if it goes down, your network users won't even be able to log in to their machines to even have basic Internet access.
Finally, seizing the FSMO roles is trivial. As long as both DCs are Global Catalogs, you don't even really have to transfer the roles if you're going to be fixing the downed server immediately anyway.
You're already in a bad position. You should be working towards rectifying it by adding the additional infrastructure that you need to eliminate all-or-nothing downtime, not throwing your hands in the air and saying "well we're pretty much screwed anyway."
Best Answer
Pretty much what alphamikevictor said. I just wanted to add that Microsoft has contradictory information out in this regard, or did. Their Small Business Server (RIP) offering provided an all-in-one domain controller/Exchange/SharePoint server, but aside from that they say:
I can personally vouch for "System shutdown will take considerably longer if Exchange services aren’t stopped before shutting down or restarting the server." In an SBS-style all-in-one environment with no backup domain controller, it can take an hour for the server to shut down while Exchange tries to contact a domain controller (which shut down before Exchange). ("Bueller... Bueller... Bueller... Bueller...")
The Small Business Server all-in-one offering has been replaced with cloud-based services.
Since the Exchange server is already a domain controller and they say, "Demoting a domain controller to a member server isn’t supported," this question is pretty much academic.