The main problem with this philosophy is power creep. If during the Vex Mythoclast meta they would just buff the other guns to compete, and then a new meta developed and they buffed the weaker guns again and so forth by now we would have weapons' time to kills down to fractions of a second, and it would turn the Crucible into a CoD style experience of whoever sees someone first wins the gunfight instantly instead of having interesting and dynamic firefights. Special weapons and one shot kill abilities being rampant are what go against this in D1, but the solution is not to buff everything until all guns are very strong. It is maintaining everything at an intended power balance which I believe the Destiny 2 beta takes a big step forward in.

I think the real issue is keeping a separate balance between PvE and PvP, so that weapons can still feel fun and remain strong against AI enemies while not having an instakill crucible experience.
But I do agree that blanket nerfs do hurt the game more than anything. If an individual gun is too strong in PvP the ideal solution is not to buff all other guns or nerf all guns in that archetype but to address the weapon's individual stats that make it overpowered, whether it be damage or ease of use or perks. Nerfs inherently are not a bad thing.