A level 8 monk is often Dex 20, but will have proficiency +3, only - +4 comes at level 9. So we get +8 for d8+6X2, with unarmed strike of d6+5. You're underestimating this basic monk damage. My 7th level monk has a flame tongue short sword and an insignia of claws as an example of a monk that gioes far beyond these assumptions.
We can make this much simpler: They should have the same attack bonuses, so the damage should be all that matters.
At 8th level, the monk is does d8+d8+d4+(3 times dexterity).
The Warlock with Hex is dealing d10 + d10 + d6 + d6 + (2 times charisma) when they have Hex available, or just (d10 + 10d + 2 X charisma) when the hex is not.
Monk is (11.5 + 15 = 26.5) versus Warlock (18 + 10) with Hex, or (Warlock without Hex (11 + 10). When you factor in those flurry of blows, which the monk can do 4 to 8 times per SR (or 2 to 3 times per encounter) in which the monk adds another 8.5 to their average damage for those attacks, which more than closes the distance.
Further - monk damage is broken down into more attacks, so there is less damage wasted on overkill, more opportunities to break the damage down amongst more foes, comes with more incidental 'no action required' benefits (such as those of the Open Hand Flury proning/pushing, or stunning), and does not need to worry about cover reducing accuracy.
All in all - and I say this having played both a monk and a warlock in the past year for significant time - the monk contributes more to damage unless you do something like a Sorlock.