I well aware that it's plastic and I'm sure your experiences are valid and no I don't have any experience with those guides. All I'm saying is that I personally don't believe that plastic in an engine compartment with a chain running over it is made to last.

If it's "stupendously brittle" to my basic understanding would indicate that material has been compromised from what the original might have been.

That's just my opinion and it really doesn't matter what I think but since the OP asked for our thoughts...I shared mine