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This page in a nutshell: No matter how many more people think first about Céline Dion when they hear "Titanic", the wikipedia article should be about the boat.
The problem with WP:PRIMARYTOPIC as written now [edit: has been modified to include "educational value" and "vital articles"] it that it mentions only quantitative factors. It is difficult to write the guideline to consider qualitative considerations in a way that doesn't make it prone to WP:GAMING, but that does not mean editors should refrain from qualitative judgements, or we could as well let a bot rename articles automatically. Popular does not always equal primary.
Note also that Titanic redirects to RMS Titanic, a good example of where precision is preferred.