You don't have to. It's still a fact.
Slippery slope is a logical fallacy when there is no chain from beginning to end. Fairness doctrine absolutely has no need for net neutrality in order to be established. In fact, fairness doctrine existed decades BEFORE the Internet. The chain of this slippery slope is not even necessary, thus it is a fallacy.
Slippery slope is also a fallacy when one does not naturally follow from the other. Net neturality is a restriction on the conduct of network carriers. Fairness doctrine is a restriction on the conduct of content publishers. The only thing that connects these two issues is the historical regulatory agency for the telcos and for the content publishers happens to be the same.
Given our modern over-reaching government, slippery slope is usually valid in a "cat out of the bag" scenario, where if you give the government power to do A, they may do related B. The problem here is that the cat is already out of the bag on both fronts: The government has established that it has the power to regulate the telcos, and it has separately established that it has the power to regulate content publishers*. There can be no valid "cat out of the bag" slippery slope argument because the cat's already out of the bag.
* The constitutionality of the fairness doctrine was very narrow and is wholly inapplicable to the Internet. The Supreme Court case of Red Lion Broadcasting v. FCC critically hinged on the fact that the broadcast spectrum was a limited public resource that is denied to others. The reasoning was that if only a few entities could be the ones to broadcast news over that limited public resource, then they could be made to use that shared resource "responsibly" in the supposed public interest. The Internet is not a limited shared resource in this sense. There is no practical limit to the number of news and commentary resources available to the public. There are already close to 300 million web sites and over 120 million blogs on the Internet, and there's room for far more than that. The term "net neutrality" has Google giving me 3,790,000 individual results of news, commentary and discussion from different sides of the issue in .12 seconds.