Fibre's a psyop man.
Think about it.
They literally turned flour waste into a health food product.
Why would putting indigestible substances into your digestive system be a good idea?
Before you say gut biome, I've got some probiotics to sell you.
Discussion
I did try & find a credible source for the damage psyllium does but I can't find anything. Apparently psyllium is amazing.
I guess I made it up.
Sarcasm is such a waste of everybody's time ...
Your argument works great on micro plastics: if it doesn't decompose in hundreds of years, basically being inert, why would it harm to have some mg of it in my body?
For fiber though I think there is a case to be made that it increases the volume of what reaches your lower intestines and I think there is scientific evidence it lowers colesterol. Clearly it's not its chemical properties but its mechanical properties.