As a Peltasta player, I completely disagree with that idea, it will make High Guard worse than before.
Before, you can block so many attacks during High Guard duration without using Shield Guard.
With your suggestion, Peltasta can only block a few attacks for a short duration and they will be vulnerable again without Shield Guard. Archers can simply auto attack them a few times and then start spamming other skills.
Your suggestion can work if it is a bonus effect. So, Peltasta with High Guard can have perfect blocks for the first few attacks and they will still have the bonus block for some duration. It’s basically a buff for High Guard.
IMC can simply make some changes for Fencer instead.
For example : a buff with an effect where each successful hit dealt by Fencer will increase their evasion or block gradually for a period of time.