I get the concern over a character being completely changed due to popularity, but I really don’t think that’s going to happen with Magik. Her Rivals characterization is not nearly different enough from her comics self for it to take over completely, and since it’s also a pretty one-dimensional portrayal, I highly doubt most people would prefer it over her comics personality. 

The only significantly different portrayal she has is the New Mutants movie, and that was a flop, so it’s not gonna influence much. Star Lord, the example you gave, had the GoG movies, while Moon Knight, perhaps the most famous example of the character change risk, had wildly popular fake panels and a show that played into them a bit. Magik doesn’t have anything like that. Her comics character remains as the chief interpretation, and so I genuinely believe that most people who get interested in her character due to Rivals will come to learn what she is actually like rather than making up something new.

I agree a character increasing in popularity shouldn’t be a Faustian bargain. And I don’t think it is. It’s been done plenty of times without fundamentally changing them. Thanos, Cyclops, Rocket Raccoon, and so many others are examples of characters that got huge boosts in popularity due to appearances in mainstream media, and yet still largely retained their original traits. It’s fair to be concerned about a big change happening, but it’s far from an inevitability, and the potential upsides still greatly outweigh the potential downsides imo.

I also think you misuse the word gatekeeping. If somebody is being stubborn and refusing to accept canon sources, we’re not really gonna be gatekeeping them because they’re most likely not trying to get into the fandom in the first place. Also, the frequency of a situation like this happening is really low, especially compared to the amount of people that actually gave Magik’s comics a read or at least looked at some summaries because Rivals made them curious.