One other dynamic effect of introducing slots to racetracks is the fallout-effect, that is on tracks who do not have slots, are now feeling the hurt as these very same horses can now ship (in many cases) 150 miles or less, race against similar horses, but for 100% more purse money.
The Meadowlands is a prime example of this. Built in the late 70's as the premier harness track, it quickly adopted the moniker "The BigM". But recently, they have had to cut out Sunday racing due to small fields, lack of entries, and competition from nearby Yonkers racetrack, and Poconos, who have slots and the powerful lure of a high purse structure. They recently dropped the nickname, and went back to "The Meadowlands".
Here in California, we are facing a challenge of horses leaving our state for greener pastures like never before.
Soon, we will probably see marginal, struggling tracks close, as my opinion is the state and federal government is only really concerned with focusing on the most efficient method of extracting tax dollars from gaming, and horse racing must face the fact that slots work 24/7/365, and require little in terms of property use, policing, and constant regulation.
Are slots a required life-preserver to racing? I would say yes for now, but the world constantly changes, and those who can adapt best, survive.
Sorry for the long answer.