No reasons to have more then 40k mag with how light/heavy attacks, shields and pet scaling have changed over time.
Before a max mag build made sense as all mentioned above scaled on mag and mag only. Now thats not the case anymore, as:
LA/HA now scale from your max magicka and spell damage
Shields are now capped at 40-50% of max health, no matter how big your mag pool is.
Pets now scale of all your stats instead of only your max mag.
So, in this day and age you get alot more with a well rounded build than all in mag.