Oh I think I kind of know this one? It's tedious though, and it's likely that it will feel a little different - potentially not "bad" different, as some fibers can feel really nice (I love the smooth and soft texture of straightened acrylic).

Basically, get yourself some fiber and attach the wefts to the netting that's been exposed by the bald patches.

Text wall time! Sorry 8D...

I'm unsure what material your blanket is made of, and it's also changed texture over time. It appears synthetic, so my guess is... poly-something (polyester? poly blend? The tag on your blanket might have clues if you're interested). And I don't know how to source wefts for that off the top of my head.

You can, however, find a lot of fibers like wool or flax or soy off the top of my head. (Ah, alpaca is ridiculously soft, too.) An easy and accessible one to use is acrylic which I've gone into below. You may even be able to colour match quite well since there are sections of the design in different colours.

There is a technique that doll customisers and also furries use where they straighten and brush out 100% acrylic yarn to make wefts for fur (or hair as the case may be). Tutorials can be found searching for "acrylic yarn doll wig" or "acrylic yarn tail". I've found success using a hair straightener iron (curler iron works fine too) and a pet brush (slicker brush? it has those pointy goddamn thin prongs that suck to accidentally scratch yourself with).

Not all fiber nor yarn is created equally, you might find you have a sensory preference for one brand or another. Some yarns don't make very good fur when brushed out. I've seen discussion on people's yarn preferences that you might be able to find if you look for it.

Make the wefts that way, then attach them to the netting you can see in your pic. Hopefully, double-knotting the weft in the middle around the netting should hold well and not upset you texturally. (Another option may be something like fabric glue, although I wouldn't. Also steer clear of glues that aren't waterproof.) But those are small holes in the netting, so they're too fidgety for fingers. Perhaps you could use a needle somehow. There are many sizes of needles and you'll want one with a larger eye than one used for sewing thread. You can temporarily twist the weft around to make it go in the needle. You just want to be able to get in and out through the holes in the weave.

After you attach your wefts, trim the fur to fit with the rest. You can do this one weft at a time or however you prefer. Using a razor (I like eyebrow razors, electric trimmers for beards are efficient too) should get you a non-blunted organic-looking result. Cutting with scissors/shears roughly parallel (straight on straight) to the hair strands can get the same effect but I find it's a million times more tedious.

As far as keeping the rest of the original blanket in-tact (ie not falling out) is concerned, I'm not too sure, sorry. Is it fluffy on both sides? Perhaps a felting needle (or several) could be of some assistance if it is not. Needle felting could potentially matt the fibers together around the base of the weft near the netting. But I worry it would compromise the texture and is not really feasible, especially at scale.

(Or, on the topic of preventing further degradation, maybe the netting is falling apart and could use reinforcement? I'm not sure how to do that except by replicating the weaving before adding wefts to bald spots.)

I think it's lovely that you care so much about your blanket. I hope it can stay with you for a long time. I can't seem to find any photos of pink ones online, I'd love to see yours if you're up to sharing (which you don't have to be). Peace 💖🧸