I disagree. There is always an entrepreneur who takes advantage of an opening or gets out before the bubble bursts. To assume that a few big players can dominate the entire market is naive and underestimates the ability of some individuals to take a risk and take over the territory ceded by competitors. Furthermore, you gotta ask yourself how horrible an executive this guy is if during the greatest recession since the Great Depression he hasn't already done all he can to maximize efficiency without ceding territory. At this point, he would most likely be acting spitefully, not in his own rational best interest, and he would be chopping off his own foot to scorn the government.
What opening are you talking about?
Businesses are driven by profits. In a competitive marketplace, where each firm faces stiff competition from the others, incentives are to reduce costs as much as possible without compromizing on quality or service or anything else (i.e., minimizing quality-adjusted costs). The Obamacare mandate is a tax - a big cost imposed on a business. It is a variable cost in respect of full time employees, but not with respect to part-time employees. To the extent that part-time employees are not of materially lower quality than full-time employees (which is a reasonable proposition in this relatively low skill/low investment service), a firm can do a better job at keeping costs down through more part time workers as compared to more full-time workers doing the same job and producing the same "ouput".
As a result, each firm in this competitive market has an incentive to substitute away from full time workers towards part time workers.
THAT is the economics and THAT is what the "entrepreneur who takes advantage of an opening" is incentivized to do, not to pile on extra costs that consumers don't give a crap about and which makes them less competitive against their rivals. Unless they are not very good entrepreneurs, but the market has a way of dealiong with them over time.
And this guy has alterantives for his investment. As a result, what he is concerned with is the rate of return on his investment in this business relatvie to what he could earn in equivalent (risk-adjusted) other businesses. And if his return is not big enough in the restaurant game any more (say because one of his largest costs - labour - just went up substantially because of Obamacare) then either he figures out a way to avoid that cost increase (as here), he figures out a way to pass that increase onto consumers (which generally requires market power), or he stops investing additional capital in this business and directs his money to other businesses with a higher rate of return (returns in the restaurant business are not that good, I don't think).
The only way your argument makes sense is if the input becoming more expensive is so important and is so differentiated from the alternative that you cannot substitute away from it without massively harming the quality of your output. You know, like dropping Green Day from a concert ticket and picking up the guy who sings at the local Starbucks on open mike night. If those were your chocies and someone was dumb enough to drop Green Day, sure, some entrepreneur would see that the cost of the input was worth it. But we're talking about wait staff, not rock starts or nuclear physicists.
Oh, and as a general matter, taxes on inputs not only harm overall purchase of those inputs, but also reduces the amount paid to the people who own the inputs (in this case full time wait staff services and the people who are full time wait staff, respectively). So the Obamacare tax reduces full-time employment and reduces the net wages paid to those employees that are maintained at full-time.
All of this is pretty basic stuff.