That assumes everyone actually knows what his objectives are before electing him, or that those objectives stay the same after election. Given how often his views have changed so far, I'm not sure that's a safe assumption to make.
Edit: Also, to be clear, if he does have legislative support because enough people support him to elect representatives with similar agendas, that's not really something for me to be worried about, as that would be no different than any other candidate. If it's the will of the people, so be it (as much as I dislike it).
I feel slightly better about this with the knowledge that the office of the president of the US is primarily a status-quo-preserving institution.
The interest groups that care generally want the status quo preserved.
If Trump is not destined to do this, then I think there is little danger he'll be elected.
Note, the same applies to HRC.
If a sufficient electoral base exists to elect Trump, a sufficient electoral base exists to get him sufficient legislative support (sooner than new people are elected, perhaps, simply by raising the prospect of electoral danger to present incumbents) to implement his policies.