I think there are a number of mitigating factors with sneak attack. Ammunition for ranged attacks, you can't gain it if you have disadvantage etc. It's not hard to get but it does take some work. As you say though, there are also some advantages. I speak as someone who plays a warlock and it feels like rogue progression would be too much damage for an at will spell attack.do they though? and the rogue gets to add a weapon die and stat mod.
I rarely ind rouges that attack 2 rounds in a row withoout 1 being a sneak attack... and I defiantly see more combats where the rogue has SA every round then ones where there is even 1 round without.
if we did this the rogue would still be ahead
1d4 (knife) +3 (dex) +1d6 sneak attack (average 9)
1d6 eldritch blast (average 3.5)
1d4 (knife) +3 (dex) +9d6 sneak attack (average 37)
9d6 eldritch blast (average 31.5)
mabe that is why at like 15th+ level there was a skip level where rogue gor +1 d6 and the warlock did not...
I think we can make eldritch blast work off something other then cantrip damage to make it stand out... right now it is multi rays.
my quick idea would be "You deal 1d6 per prof and at level 6 can split them into up to 2 beams, and at 12th up to 3 beams, if you split the beams 2 beams can't targeet the same target"
then you can have an invocation that adds cha mod to damage of each bEAM