I don't think "average" lyrium veins can 'feed' entire civilization, as well as ginormous tree. There's something else going on here... But then it seems that any lyrium vein is connected to some sort of Titan.
As for Titan not minding... well, maybe for it, creatures like Sha-Brytol are either too small of a concern, or they at least have a useful role, in all heir zealotry or mindlessness, as they keep everyone else away.
Still, the Titan appears to be in some sort of stasis or sleep phase. The Breach caused it to 'stir', after all - hence the earthquakes... Not sure I want to know what it would do if it fully woke up
....Was the Titan waking up what consumed Arlathan?
Anyway... the suggestion that Thedas may be going through the natural cycle of death and rebirth from those speculation is something I wouldn't dismiss too quickly.
Is it possible, that something went wrong with previous cycle, and it created the Blight? ... Or is it that elves (?) constantly prolonging their civilization, and therefore the cycle, caused at least one of Titans to become corrupted out of sheer strain? Do the Titans willingly work with elves or their gods? Were they enslaved, or maybe imprisoned and used as a power source? One of the old elven texts found in ToM does suggest that some elves were preparing to hunt "pillars of the earth" and that they will 'make the earth bloom with their passing' - in light of what we saw in Descent, I wouldn't be surprised that they indeed tried to 'hunt' a Titan to rejuvenate an ailing world... Question is - did they succeed? And was the price for it a fall of their civilization?