No 16x16 MIDI interfaces I'm aware of, but there's the already-mentioned Edirol UM-880 and as you know MOTU do currently offer several USB 8x8 MIDI interfaces (ime both Roland/Edirol's and MOTU's MIDI gear has always been very good and well supported driver-wise).
Dunno how many MIDI devices you need to use all at once, but you might be able to get by with a single 8x8 interface along with a pair (or more) of 8x8 switchers/patchbays like the Yamaha MJC-8, with patches set up for different routing arrangements. Might be a mess of cabling on a desktop, but could be made somewhat neater in a rack.
Then there's always the old fallback of daisy-chaining via MIDI THRUs, which might save on the number of MIDI ports required.
I'd venture the decline in MIDI interface offerings is due in part to the fact that a lot of current MIDI gear now uses USB for interfacing to computers, just like most current MIDI interfaces do, obviating the need for a dedicated MIDI interface.
Edit: Almost forgot, there's also the M-Audio Midisport 8x8 which although no longer in production is still supported afaik. Was M-Audio's top of the line interface, competing with MOTU's MIDI Timepiece offering, That said, haven't used any recent M-Audio interfaces (not since the old Midiman Portmans) so dunno.
post edited by Goddard - 2012/08/23 05:50:19