[hcobb]

Assuming a 30 year adventuring career at 50 XP and 1% mortality per month.
Year 1: 600 XPs gives 35th Att 200 XP change, 89% survival.
Year 2: Another 600 XPs gives 37th Att, 79% survival.
Year 3: Another 600 XPs gives one skill slot 100XP change, 70% survival.
Year 4: Another 600 XPs gives one skill slot 200XP change, 62% survival.
Year 5: Another 600 XPs gives one skill slot 300XP change, 55% survival.
Year 6: Another 600 XPs gives one skill slot 400XP change, 48% survival.
Year 7: Another 600 XPs saved up for 1000XP change, 43% survival.
Year 8: Another 600 XPs gives 38th Att, 38% survival.
Years 9 to 14 gain 6 skill slots, 18% survival.
Years 15 and 16 gain two more skill slots and 200XP change, 15% survival.
Years 17 to 21 gives 39th Att, 8% survival. (Character is at starting IQ plus 12 skill slots.)

The next ten years aren't enough to get to 40th Att.

IQ 15 wizard would take five years to fill his Mana under this system while gaining nothing else.

Of course true adventures should be much more deadly than this.