Dead Astrea snail

I have found a dead Astrea snail today on the bottom of the tank.

It was a big snail, I had it for more then 18 months - from the
first group of snails purchased just after the tank cycling...
Its shell was covered in whole with coralline algae, partially pink
partially maroon/red (velvet like). I have found it two days ago
still alive but laying on the bottom helpless with small pieces
of rock attached to his foot - he could not reverse itself.
I helped him to touch the rockwork and it attached himself but
did not travel like other snails rather sitting in one place two
days. This morning I found his shell scavenged by two bubble-bee
snails foraging on his remains... This is the first time any of
my astrea snails died so this raised some questions:

Do you know how big they normally grow (measured in shell diameter)
and how long do they live in captivity assuming quality conditions?
Would you blame his death on a long time he could not eat helpless
or rather some other factors like his old age, attack of some predators?