I just got to thinking, that if this were happening in the 70s, both the Thrashers and Coyotes would probably have been contracted. Sports is a very different business these days. Leagues don't want to contract teams except as an absolute last resort.
I agree with your point, but I wonder why that is so. If the NHL can't find a legitimate buyer for Phoenix next year and Glendale stops funding $25 million of their losses (both quite possible), then from a hockey standpoint I don't see what is so bad about contracting the league to 28 teams by taking down either Columbus or Florida along with Phoenix. Sure the optics are bad, but I can't imagine that still trumps the money that existing owners are losing annually to prop up that franchise.
The best reason I can think is that if teams "never" fold then that raises the price of the weakest franchises because prospective new owners can be assured that, barring bankruptcy, they can always recoup at least part of their investment.
The league can always expand back again in the future, and expansion fees are likely to be higher than relocation fees.
I put the chance of the league contracting 1 or 2 teams at 25%. Too many teams are in trouble, and I think the alternative locations (Kansas City, Quebec, Toronto area) all have issues.