Mulcair was anything but strong. Everyone knew going into the 2015 election that the Orange Wave would recede. Mulcair's whole mission was to salvage enough of the wreck to make it not look downright embarassing. To do this he started softcore pandering to Quebec sovereignism, culminating in an utterly humiliating moment in the debates when he arrogantly grilled Trudeau about the threshold definition for Quebec sovereignty to happen:
https://www.youtube.com/watch?v=zwtU4KGGYVUWanna know how the NDP blew the majority polls early in that election? Yep, it was Mulcair, who was anything but strong.
Politics in its modern form is often a full-time job. To climb the ladder often means one must start early, yet this job is anything but stable. Most regular families can't really afford the risk or downsides. Political dynasties have the advantage of existing connections, as well as inter-generational resources that aren't available in other families. In this video https://youtu.be/XqjQ6gqgi0w?t=41 you can see Trudeau Jr.'s behaviour was clearly groomed for attending public functions. This was a headstart that many others don't have when they go into politics well into adulthood. The same phenomenon is present in Japan, just at a much larger extent. Social connections is an asset without form, and naturally it can get passed down generations like other assets. Why abandon them when they can be used advance your career. Unless we adopt ancient Athenian procedures of ostracism, political dynasties will exist. And ostracism will not happen because it was idiotic and backfired badly.
Justin Trudeau had been a MP for 5 years when he became the Liberal leader. Their party was in crisis mode after a decade of consecutive seat losses. Trudeau was the button behind their "break in case of emergency" glass, and it worked for them because the same effect resonated with the voter base.
Modern politicians in developed elective democratic countries aren't benevolent despots, and no single person's brain has the capacity to process or react to everything going on in the world at this day and age. This means they have lots of advisors to handle most of that processing and decision suggestions. The relief of this burden means a good modern politican doesn't have to be good at policy - they become another type of influencer whose first and foremost focus is charisma. Zelensky is another example.