I'll try to give an answer that doesn't have any of the "bungie has a hardon for warlocks" childishness.

Let's distinguish between super game and neutral game. Super game is what you do with an active super, neutral game is what you do without a super.

Sunsingers have arguable the least kill potential in their super. I'm not saying you can't kill with it, however compared to other roaming supers it's subpar. Slow movement, only a single 1-shot kill if you have a charged melee, and needs 2 firebolts to land to kill. Using selfrez is hard-countered by revive sniping or shotgun/fusion + melee. Using radiant skin gives an extra 5% dmg resist only, which is nothing special. Compare to Stormtrance or Arcblade (melee registration issue aside), which are better in terms of kill potential.

So it makes sense to give a stronger grenade/melee to sunsingers, if only because their super game is weaker.

Now, if you look at Bladedancers, their neutral game is amazing. That's primarily thanks to the Quickdraw perk and Blink. Having a solid super and the best grenade alongside firebolt made them the arguably OP class they were in Y1.

Stormcallers have what is now arguably the best super, and longest-range melee. However, their grenade options are average. They actually have to be, because the perk that chains extra lightning when enemies are hit by grenades can be beastly. Look for the recent videos of 1 stormgrenade = 5 kills that showed up on this sub. Imagine this with pre-nerf arcbolt, which hits instantly and hard and chains and has great range of chaining.

A final point is that using VF/ToF firebolts locks you into specific perks. So having the buffed firebolt makes you give up on extra Radiance duration, or having an extra grenade. Arcbolt was great in its default form.

Will all the above points convince you? Maybe yes, maybe no. You can probably find a counterpoint to all of them, if you really tried. However, keep in mind that you can't look at only one aspect of the game (Arcbolt grenade vs firebolt) and not consider how it would impact the overall game balance.