Your suggestion does not generally accomplish that either Look at the math, you just replace a d8*x firebolt (avg4.5*x) with d8+int mod (generally flat 4 or 5) or 2d8+int mod.
I didn't replace 1d10 with 1d8+stat mod, I replaced 2d10 with 1d8+ stat mod...
2d10 min 2 max 20 average 11
1d8+4 min 5, max 12 average 8
I then replaced 3d10 with 1d10+ stat mod
3d10 min 3 max 30 average 16
1d10+5 min 6 max 15 average 11
I then replaced 4d10 with 2d8 + stat mod
4d10 min 4 max 40 average 22
2d8+5 min 7 max 21 average 14
BUrning hands does 3d6 fire and magic missle does 3d4+3 auto hit force (I think we can all agree auto hit + less ressited type is better)
3d6 min 3 max 18 avrg 10 (or 1/9/5 with save)
the only time my average damage in my version tops better then burining hands is when it hit and burining hands misses the save, at 17+ level... but even still the fire bolt can miss 0 damage, and burining hands is save for half so 1/9/5 fire damage on a 'miss'.
Cantrips don't exceed leveled spells at times because they are too good, they exceed them because low level spells no longer scale with caster level and because spell slots no longer accumulate at the more generous rates that would have supported upcasting better.
edit: I am sure someone can make even better math... but its a darn good start for an off the cuff answer from a non game designer