And if such marines were to become tutors or mentors AND they were immortal, then surely we could have some marines live past Dante's 1100 years. They may not be the norm, but surely out of the active 1,000,000 marines out there, some mentor or tutor would be rather old, no?
I posited that they would likely be doing that IF they lived on, crippled and combat-ineffective. I also offered that it's highly unlikely that they would live on after such injuries that would cripple them regardless.
Put it this way. We know, via fluff, that Astartes can survive losing limbs (heck, the Iron Hands PREFER bionic limbs), blindness ("Brothers of the Snake"), etc. Given this, what kind of injury could render them incapable of fighting? Probably some sort of massive torso trauma. Surviving that would be incredibly rare.
Besides, the whole 40k universe is a mass of myth, superstition, and ignorance. Dante and Grimnar are known because they are legends in their own time. Why would anyone know about some poor rank-and-file Space Marine who got crippled during his first couple of centuries and has been doing nothing but sitting at the Fortress Monastery since then?
And I'm not 100% that each company is made of 10 squads? I have to run to school right now, so if someone would be kind enough to look it up it'd be appreciated.
They are. Codex Battle Companies have ten squads: 6 Tactical, 2 Assault, and 2 Devastator. The Sixth and Seventh Reserve Companies each have 10 Tactical squads that also train on combat bikes and land speeders, respectively. The Eighth and Ninth have ten squads of Assault and Devastator Marines, respectively. The Tenth has ten squads of Scouts.
Of course, that's theoretical combat power. The reality is that they probably already have open slots, to reflect battle losses, etc.