With all due respect for Sir Ian Holm, he is now physically at least 26 years older than the 40 to 50 year old Bilbo. The Hobbit is a journey movie just like LOTR and walking around in those Hobbit feet, and being the central character of the films would mean a pretty merciless shooting schedule with hours averaging 14+ a day, often in inclement temperatures and weather.
They de-aged Sir Ian McKellen and Patrick Stewart in X-Men the Last Stand (I only watched the 5 minute opening just for that scene) and it was not THAT bad, but it was a little unnatural looking. Yes, I'm sure its possible to do that to Holm for an entire film, but imagine the costs and post production time. This still doesn't help Holm with the physical nature of the role. I just can't see it happening.
I think the best we can hope for is GDT casts a relative unknown who surprises us with his ability to bring the sort of warmth and humility that Holm brought to Bilbo. All this talk about well known actors/celebrities is just typical of the early pre-pre-production period of a film,. The media won't thrill their market if they suggest John Smith will be playing Bilbo. So I'm sure Guillermo, who is an accomplished director and knows what he is doing, will make the right choice for everyone involved including the fans. I hope he was not right about having been handed the keys to the nuclear button. That would be unfair for a director to feel they are in that position with the fans.