I think a eff7 is worth every penny, I still use max picks along with my eff7, I just expose the ores with my eff7 and then fort them and keep moving, I have made my money back greatly with my eff7.
Honestly, if you have decent armour on you cant really die with it, mobs are that big of a deal (when you find one). Lava isant either if you have pyro+ or even max fire prot/prot you can swim in it like water.

 

A quick point.... prot4 armor will make you ~nearly~ immortal to mobs. Put thorns3 on them and make a macro to automatically /repair all, and you're unkillable, they'll die before killing you.

Get lava+
get water+
get fly+

the only thing that can kill you is void or players in pvp.
edit: just remembered the old captain 'rank' prevented void damage...

Now to the math.

Code:

Eff 5's make generously 10k on a fresh restart. They cost ~about~ that much for a fort/silk combo.
y1=-10,000+10,000x
Eff 7's make generously 25k on a fresh restart. The costy about 900k.
y2=-900,000+25,000x
using those two formulas we'd set them to be equal as shown:
y1=-10,000+10,000x
y2=-900,000+25,000x
y1=y2
-10,000+10,000x=-900,000+25,000x
Simplify (subtract 25,000x from each side and add 10k)
-15,000x=-890,000
Simplify (divide each side by -15,000)
x=59.33

Conclusion: It would require hard core mining for 60+ hours to make the same amount of money with an eff7 as an eff5. Is it worth it?

That's up to you.