My mate Vince on YouTube (his channel not my actual mate) has some good videos that show how to replace it - with plastic or metal
He points out that the reason they are plastic might be a deliberate flaw - if you drop your switch it's safer to lose a joycon rather than have it stay in the console and send the shockwave through the entire switch - just a thought
I had the same problem and got it fixed! I called the Nintendo Contact number (855) 877-9099 (on their website) and I told them about my problem. The phone call only took about 10 minutes. Their customer service is also very friendly. I sent my joycons on Feb 21st and got them back March 1st. They send you a UPS label via email and you just have to ship it to them by either scheduling a pickup or dropping it off at UPS. You should get yours fixed today because today is the last day for 1 year warranty.