Earlier quoted context omitted.
AFAIK, the US and UK value Diego Garcia because currently there aren't geographical alternatives for that base. Where else could they put it that would have the same benefits?
The lease expires in 2123. The militarily strategic landscape then is pretty much unknowable. To a 1925 (99 years ago ) military force, the Diego Garcia airfield would have had zero importance.
I bet that's what the UK thought about Hong Kong in the late 1800s, but when 1996 rolled around I think they (and many HKers) would have liked a longer-and-99-years lease.
While geography isn't quite destiny, it is fairly important, and having a random rock in a place where there are no other rocks will always be useful IMHO (unless we perhaps develop teleportation).