This will not be sufficient to support disabling inventory tracking. Spree will continue to decrement the inventory total and there is a separate validator to check whether a product has supply available (see Spree::Stock::Quantifier). So, with this minor injection, the customer will still not be able to order the product once the inventory is below the requested quantity (i.e. inventory tracking is still in effect).

I am looking into options for this, but it may be fairly significant to cover all of the bases to disable inventory tracking effectively.