To be fair, model packs are mods (specifically, a sub-category of mods), but just as Urby says they're very unpopular, especially for a game as old as Half-Life.
Model packs, and other content-replacement packs such as texture replacement and audio replacement, don't bring any new experiences to the game. The game might look or sound different, but the gameplay is identical and any novelty introduced with the pack quickly fades to boredom.
However, if you made a series of maps that fit the mood of the models, and bundled all that together, now that would be a far more interesting mod!