Just because a lot of magical creatures might have to age along with their magus that doesn't make it that bad a deal. Magic critters get killed in the wild all the time - by magi, by other critters, by accident. If it's a choice between definitely dying in 150 years, or probably getting killed violently within the next 10 years but theoretically living forever, what's the right choice? Those Might 40-50 creatures aren't likely to get killed, so yes, those creatures are less likely to be interested in becoming a familiar.
But the rules for familiars were written long before RoP:Magic came around, where magic creatures weren't necessarily immune to normal aging. As written in core, it's beneficial to the animal as it assures that they won't age to death before the magus does - a magic mouse would only live a few years if it aged like a normal mouse, which was the assumption that RoP:M turned on its head. Familiars also didn't have the might related learning penalty until RoP:M came around, it really changed the rules significantly.
Plenty of magic animals probably do have Susceptible to Deprivation and Age Quickly flaws, making them age like normal animals. The familiar bond is very attractive to such creatures. As for the ones that are ageless, they should probably remain ageless - those rules wouldn't apply to spirit, daimonic or faerie familiars either.