Fleetwood Mac is a different band than Eagles or The Beatles. Both Eagles and The Beatles had the same people as primary songwriters and singers throughout their history. The current Eagles lineup has replaced Glenn Frey, through necessity, but done so with his son and another respected musician. Eagles do have a history of adding people who wrote and sung songs, but these were more peopl who sung one song an album or so.
Fleetwood Mac has had a history of the primary songwriters being replaced or added to. Sometimes successfully, and sometimes not. Christine McVie is the closest they have to a continued prescence, but even she wasn't present for a number of FM albums.
Hence, I think it's very different Neil joining Fleetwood Mac than it would be if he joined The Beatles, Steely Dan, or even Eagles. If Neil joined any of those bands to 'fill in', then that's what he'd likely end up doing.
With Neil joining Fleetwood Mac, there's the real possibility that there could be a new album with Neil playing a major, perhaps even the biggest role, in songwriting and singing.
I'm not too interested in the FM concerts as I don't pay hundreds to attend concerts. I'll buy a Blu-Ray or digital download if one is released. But, if there was to be a new FM album with Neil playing a major role - now, that would be interesting.