1. The insurance industry will decide on "reasonable" either directly or indirectly.
Wellpoint (Anthem), Cigna, and Aetna (among others) will fight like hell to keep that control directly, and Congress has a recent history of caving when industry decides it wants to regulate itself. Even with TARP, a direct government giveaway, the financial services industry demanded the right to pay whatever obscene bonuses it wanted, the banks reserved the right to hoard taxpayer cash instead of lending it out (like they were supposed to) and the Fed still doesn't want us to know what banks got that money. The arrogance of the financial sector, of which insurance is a large part, is astounding, and it's not going to change.
But, even if, through some odd set of circumstances, Congress gained the power to decide on what "reasonable" is, so many individual members of Congress are so compromised as to be useless. They are so compromised that the Senate bill now being forced on the House was actually written by Liz Fowler, former VP for Public Policy and External Affairs at WellPoint. Max Baucus alone took about $1500 a day for 5 years from health insurance companies:
http://www.mtstandard.com/news/state-and-regional/article_6720e645-fa6e-557d-baf7-4baa76739593.htmlChuckyGrassley received nearly $2 million and as a whole:
"The sector gave nearly $170 million to federal lawmakers in 2007 and 2008, with 54 percent going to Democrats, according to data compiled by the Center for Responsive Politics, which tracks money in politics. The shift in parties was even more pronounced during the first three months of this year, when Democrats collected 60 percent of the $5.4 million donated by health-care companies and their employees, the data show."
http://www.washingtonpost.com/wp-dyn/content/article/2009/07/20/AR2009072003363.htmlSo in the end, the health insurance industry will continue its practice of giving millions to elected representatives to prevent them from hurting the industy's profits. If Wellpoint has to spend a little more, they will pass that cost onto us.
2. Market incentives will be extremely limited. (That is why so many wanted to public option.)
The national health insurance market is dominated by the following insurers: United Health, Wellpoint (Anthem Blue Cross), Aetna, Cigna, Humana, and Kaiser. With so few major players, competition is limited, and there are often "gentleman's agreements" to keep prices at a certain level. In other words, they will not compete to undercut each other because they are all big enough to have a lot to lose. Smaller companies in the exchange are often local (specific to a state) and cannot compete across the country. A larger company can, and if Wellpoint, for example, wants to get rid of the local competition, it could undercut the competitor in a single state (while keeping profits high in all the other states) and destroy a small insurer. Any real competition to the big companies, therefore, is reduced by the limited access of smaller companies to markets, and the threat of undercutting and destruction if a smaller company decides to really compete within a state.
The one REAL competition for the big dominant companies would be a public option, because the big guys couldn't push the government out of business and couldn't undercut the government since the government doesn't have to make a profit. That is why activists believe that the public option NEEDS to be in the final bill to provide some cost controls and some checks and balances on insurance company behavior. If Wellpoint caps a cancer patient with a annual limit ("reasonable" of course) and the government public option doesn't have such a cap, Wellpoint will lose a incredible number of customers. That's the fear the insurance companies have which is why they so DESPERATELY have fought the public option at every turn.
You mention "a large exodus" if an insurer does something nasty--this is the theoretical market forces at work. The problem is where does that exodus go if there's no public option? People's choices of insurers are limited because health insurance companies (like auto insurance) work within state lines, and certain large insurers have high market share (if not all of it) in certain states. For example, in California, Wellpoint (Anthem Blue Cross) has 57% of the market share, and California is a huge state. In some smaller states, Wellpoint has 80% of the market share. And the other companies sharing the other 43% or 20% are mostly others of the big dominant companies. There are not always places to go, and the large companies will not undercut each other, so you will end up getting similar coverage for similar money.