Ronnie Wood, who has played guitar for the Rolling Stones since 1975, only officially became part of the band/partnership in the the 90s, for perspective.
Plenty of popular bands operate this way- dependent on who owns the name- so I've never understood why it's such a big issue for this band.
OP is misleading anyway, if Mick hired musicians for a project while the Rolling Stones were still active, then yes it'd be a Mick solo record. However, if Keith Richards and Ronnie Wood left the band and Mick had rights to the name, and hired guitarists they'd be the new guitar players in the Rolling Stones...
Edit: Just to clarify this just is me thinking objectively. My opinion on Chinese Democracy would have been the same regardless of whether it was under Guns N' Roses or a different name