Who summoned me? (Went from trance -> opera/pop ->c-pop/j-pop -> prog metal/metalcore/post -> v-kei -> k-pop)
TBH, he could have genuine interest and has "Jimmy Fallon" syndrome (compensating for a generally "dry" personality), but it's also likely the ad revenue/views. Even the most mundane of videos with K-pop content (or even the mere mentioning of groups/idols) will summon the stans. With everyone trying to "make it" on YouTube, the amount of attention K-pop has gotten has been a godsend to small channels/creators. That being said, genuinely liking the music and wanting to grow an audience are not mutually exclusive.
Now, I personally don't watch MacRyanMac regularly, but he piqued my interest b/c he comes from a somewhat similar music background as myself. My observations on the music side of things lead me to believe that he shows interest, but purposefully plays his reactions REALLY safe to grow his audience. However, what's more telling is the way he constantly squeezes in videos that involve popular topics & trends and constantly retweets larger creators. Which means he's just using this common method:
Make videos about/utilizing general YT trends to build an initial, albeit small, following.
Hone in on some large bands/trending topics to develop an audience. Alternatively, spill your guts on camera and turn yourself into a drama channel.
Mold every single one of your videos to user comments/suggestions.
Keep a steady stream of content based on said comments/suggestions for eternity w/o offending your audience.
Additionally, keep up with other SNS platforms and show up on larger creators feeds/comment sections.
Also, release a ~10 min vid at least every other day to stay afloat. Rev up those buzzwords and hashtags.
He might listen to some of these bands every now and then in his spare time, but the large coverage indicates that he's mainly accommodating to his audience. Those who find genuine interest in a genre will typically carve out their niche of artists w/ similar characteristics and then bounce around, his broad-brush artist selection doesn't indicate that (This might come off a little gatekeep-y). BUT, you can't build a large channel based on only your interests though... Which leads us back to the start.
The only way to have decent growth on a YouTube channel in this day and age is to pander to as many people as possible. People typically watch reaction videos to validate their own opinions b/c it makes them happy. Small sketch comedy, animation, and blog channels of the past are simply not viable on YouTube anymore b/c of how content dense YT is now. Some might make it, but why do that instead of the method above? It's not like it's "wrong".
We still haven't gotten to the making a living on YT part either, lol.
(This turned into a YT schpiel, lol)
TL;DR: He's interested b/c he has the patience to do what he's doing, but he's mainly doing it for the views. He capitalized off of his spike in viewership after his first react video to Babymetal which was trending at the time.