To really look at it--you need to see things from a historical perspective:

1st-2nd ed: Scatter Laser was whoppingly cool. 6 shots at St 6, 60" range. It was hella effective.

1st-2nd ed: There was no such thing as a Starcannon. Eldar used Plasma weaponry like everyone else.

3rd ed: Scatter Laser gets the huge (and I mean HUGE) nerf: 36" range, d6 shots. That's right...33% of the time it was less effective than a IG multilaser, and only more effective 50% of the time. The safest military job in the eldar warhost was to be a scatter laser crewman...

3rd ed: Starcannon: This was a new weapon for the eldar, and it quickly became the pride and joy of eldar farseers everywhere. (There was no "autarch" listing before 4th ed.) Str 6, AP 2 and Hvy 3. 3 shots, consistently killing MEQ/TEQ (before thundershields got to be 3++ period) when cover wasn't so readily available.

There was no reason to take anything BUT a starcannon in 3rd ed eldar, save for a few bright lances here or there to deal with enemy armor. (3rd ed fire dragons were only Str 6 melta back then.)

4th ed. saw that there was a need to switch things up. They made the Scatter Laser a Hvy 4 weapon, to be always better (slightly) than a multilaser, and more attractive for at-range firing than a Shuriken Cannon. The Starcannon rec'd an arguably needed, but necessary nerf to Hvy 2.

4th Ed still favored the starcannon slightly because cover was still not as prevalent as in the current 5th edition paradigm.

Now, a 4th ed codex in a 5th ed world:

Yes, Starcannon still bypass MEQ armor quite reliably, at 50% of the shots.

However, 4+ cover is so readily available as to potentially make the Starcannon the "scatter laser of 3rd edition."

Eldar shooting in the 5th edition world really relies more on volume of fire than quality of shots hitting the target. It's better to force more save rolls than have fewer shots that still get a cover save.

So, to my way of thinking, the Scatter Laser is back in the running over the Starcannon, but it still does cost more than the basic Shuriken Cannon, which is Hvy 3 and 24" range.

Ultimately--take what you want--just be aware of how your choice works within the current paradigm.