Slippery slope arguments are absolutely legitimate when (a) dealing with policy proposals in relation to difference from a perceived "normal" situation, (b) discussing issues in which factions that wish to push the status quo toward an extreme endpoint manifestly exist and exert influence, and/or (c) there's a demonstrable history of incremental expansion of similar policies' scope and effects in evidence.
The unsaid half of the example, which I assumed was understood for people on HN, was that income tax was initially presented as a small tax on the very well off which now impacts almost every earning American.
Judging by the timeline, it seemed like initial low rates had more to do with it being a new system, and less to do with some kind if "we will only do this to the rich" promise.
Also, they tried implementing income taxes previously, had it struck down by supreme court, then all the states got together to pass an amendment to allow income taxes. So, it was a pretty strongly supported change.
Honestly, I can get on board with "government tends to only grow in scope" point of view, I just don't think income taxes illustrate that point at all.
