A LFD using fel magic is like a fire elemental suddenly casting water spells. That is literally what being a light forged is all about…submitting yourself to the holy light and becoming a living being of holy magic energy.
If Illidan didn’t resist the giant wind chime light forging him, he wouldn’t be a demon hunter anymore and almost certainly would have been either destroyed by the process or the fel magic and transformations it had on his body eliminated and him back to his original night elf self and likely still a powerful mage or at least a mage with an affinity for war glaives. Even in a world where regular draenei warlocks exist, becoming light forged would have purged fel out of them or they would have just been killed by the light for being the demon-allying heretic they are and anyone willing to undergo the process to become light forged would almost certainly be so radicalized and such strong faith in the light that they would never think to stoop to drawing on fel magic.
The other big problem with the argument that “my character is special” is that they canonically aren’t until much later in the story. Canonically, you start off this game as a nobody with massive yet to be realized potential. Warlocks, like death knights, make more sense to be a hero class or a class that one becomes later in their adventure strictly by their canonical impact and how the world sees/treats them. An uppity mage apprentice who decides to consort with demons and draw on fel magic as a shortcut for power would have been ousted from mage school…not just moved to a new track.
But it’s obviously clear Blizzard doesn’t put much stock in the actual RPG elements of the MMORPG they’ve built. Their choice. Anyone whose been playing this game for awhile has seen the writing on the wall long before now. The lobby style players are a higher target demographic than RPG players hence the strong pushes to the game to more and more make this game in that style over a traditional RPG.