Development Platform Availability
Can ARM be trusted for deployment on servers, when there aren't even any ARM development platforms around?
That's a good question to ask. That is a gap in the ecosystem.
But if you've got an ARM cpu that's powerful enough to put in a server, then clearly non-rack-mount motherboards can also be developed for it.
Also, while code written in C is at risk of biting you if run on different hardware, code written in FORTRAN or Pascal... not so much.
So I don't see any problems with making the transition gradually. While x86 has a big lead, and some of its competitors have fallen by the wayside - no one is expecting SPARC or PowerPC or Alpha to come and take the world by storm any longer - I think it's too early to say that the x86 lead is forever.