People are throwing fits about this minute change. But I think the underlying fear is that this is part of a "slippery slope".
That this is a step in tandem with the casual momentum of reddit as a whole. That it's a sign of things to come. Slowly but surely over the span of years reddit will slowly implement changes until its aesthetic is unrecognizable.
I can empathize. Little changes like this do make me worried because I do care about reddit. I'm not sure that's a favorable thing to admit, but I'm on reddit everyday and I value that entertainment.
Some things to consider though -
Subreddits still have control over their own layout. If a moderator wants to revert to the old text size they can do that. Among all the other visual choices they have. The autonomy that subreddits have design wise is something that I think helps guarantee reddits continued sense of self. Because each subreddit is like it's own little world. Reddit in that sense is almost like a webhost for individual subreddit domains.
And to not be so reactionary, or paranoid - I think one can color their perception with optimism or pessimism. The optimistic look could say that reddit made such a small change because they are trying to be careful. It's just a tweak because they are mindful of how important the aesthetic is to a userbase this large. So from that perspective reddit isn't trying to slowly turn this place into Fisher Price. The slowness is actually a sign of measured deliberation. But that's a tough sell. Even I have a hard time buying into that all the way as I type.
But I take stuff at face value. For me - reddit is still as usable and fun as it's been.
Hell, over the course of writing all this I stopped noticing the difference in text size and completely adjusted to it. I think if you use reddit enough you will get used to this text quickly and forget what the old text looked like.