The longer the network runs, the more resistant it becomes to takeover, since the older miners become trusted, not based on how many coins they own, but how much mining they have done.
I think this argument is flawed. You assume that someone is more trustworthy if he has done more mining in the past. Actually, this makes an attack easier - the attacker needs less than 50% of the computing resources if he employs his resources for a longer time, thereby boosting his trustworthiness.