I'm not a game developer, but I am assuming that if you iterate on gameplay too long you have less time/money to develop high fidelity art.
Assumptions:
* Lower poly models and lower res textures can be produced faster
* Gameplay starts unbalanced/poorly paced/with a mix of good and bad mechanics and is then improved iteratively
* Hiring more artists costs money which results in hiring less devlopers/designers (or vice-versa)
* Art and gameplay do not compete for staff time (artists vs designers/developers) however due to an almost-critical path they do compete for completion time.
Assumptions:
* Lower poly models and lower res textures can be produced faster
* Gameplay starts unbalanced/poorly paced/with a mix of good and bad mechanics and is then improved iteratively
* Hiring more artists costs money which results in hiring less devlopers/designers (or vice-versa)
* Art and gameplay do not compete for staff time (artists vs designers/developers) however due to an almost-critical path they do compete for completion time.