Whyte and Britton had a bit of a mare no-one can deny that, but they took over an appallingly bad shambles left by Gerry Collins, McNamara is taking over a reasonably settled young team, McNamara is going to be the manager instead of the co-op that W&B had, so apart from their similar level of experience and age, there is a bit of difference.
There's no guarentee an experienced manager will bring success. Look down south, Eddie Howe at Bournemouth, look how well he did aged 32 or something. Then there's the guy at MK Dons, can't remember his name, don't even know that he's 30 yet, got them in play offs.
I'm quietly confident McNamara will be a good appointment for us, and the first step into a successful management career.
Edited to add: anyone else had their armchair celtic fan friends already asking to come along to odd game next season to "support wee jackie" etc. All for that if it gets few extra bums on seat some weeks. He seems very well thought of by the tic didn't know he was so popular.