In terms of Glenn IMO it goes without saying that he's beyond irreplaceable within the band; as their leader, a singer/musician, and the force that drove them. The Eagles book is closed and as sad as Don's words make me I'm glad he has addressed their future so promptly.
However I actually don't think at this stage of their careers they would've replaced any of the current Eagles members. An example: if Timothy had been unable to recover or perform after his cancer treatment a few years ago IMO that would've been the end of the band (thankfully it wasn't). I believe this for a few reasons not the least of which is Don and Glenn always strove for authentic perfection and giving their fans an amazing show. This took a lot of work and achieving band dynamics/balance was not an easy task.