When a concessionaire optimizes it's mass VS VIP mix, it shouldn't impact marketshare overall in their quest to optimize it.
Simply put, mass is low volume, high hold and VIP is high volume, low hold. When the mix is optimized, the company will produce as much revenue per table on mass as it does on VIP. If one class of offering becomes more profitable in terms of GGR than another, a company will shift the emphasis to optimize overall GGR per table.
Over the past year, premium mass has been where tables have done the best, so most of the resorts are shifting tables to that category. Venetian renovated and opened a large amount of square footage for precisely that purpose.
I would caution people when looking at the metrics, however, because many of them don't break out premium mass vs mass-mass, and the surge in premium mass is making overall mass look better than it really is in terms of margins minus incentives. Similarly, some of the recent "swoon" in VIP has come from companies re-classifying low-end VIP to the premium-mass category... SJM has been most active in such re-classifications in order to alter the street's impressions of their product mix.