AFAIK you'd need a projection for each origin/destination pair, so it's not exactly convenient. A globe is by far the best tool for such a job.

But globes aren't really that practical for navigation. You can plan your voyage, but when you're at sea, how do you make sure you're going in the right direction? Mercator has the advantage that you can use your compass and as long as you stay on course, you'll arrive at your destination.

Keep in mind: that's 500 years ago, not 50 years ago. There's no question that we have better tools today but the question was why the mercator projection was so successful it became the default. I think "it was super useful at the time because it made navigation easy, reliable, and available to everyone, at any any weather" is a much more compelling explanation than "the would-be colonizers spread it because their home-countries were enlarged and they liked that and used it to oppress the equatorial countries".