They already sort of have though with how loadouts are set up in 2 to be fair.

That was a happy compromise. In no way does that somehow show that they "picked" PvP over PvE.

Not really in my opinion. It was changed because (This is from an article but sums it up well) [quote]The PvP Crucible never quite achieved an ideal balance between those three weapon types, and things became particularly lopsided when balancing primary vs. special guns. In particular, it was too easy to dominate in Crucible by running around with a shotgun.[/quote] and then [quote]Between 2014 and now, Bungie tried to balance Destiny PvP in a number of different ways. They made shotguns shorter-range, and tried lowering their damage. They slowed the aiming on sniper rifles and increased the zoom on short-range scopes. When those tweaks didn’t work, they went after ammunition, making it much harder to get sniper rounds and shotgun shells. Players cried bloody murder throughout the entire process.[/quote] But instead of going. "Okay we're gonna limit you to 2 primaries and a special or heavy in PvP" Then went "Okay to balance for PvP we're taking away a fun slot and making it so you have a DPS primary, a shield breaker primary and a heavy or special" TL;DR There's really no reason for why they couldn't have done both and not just taken away a slot for more interesting weapons from PvE.

To be fair, the game was not able to be balanced (IMO) with the shear amount of weapon archetypes and trying to balance PvP and PvE. It was set up for failure from the jump.

It wasn't balanced. For PvP, But what the best compromise for it now would be is take the 2 primary loadout they have now since it was made with PvP in mind and have that used in PvP then have the original Primary/Special/Heavy be used in PvE.