Glad you liked it and thank you for the screenshot 
Thanks to this post, I discovered CodePen.
When I will have time, I plan to use it to build two javascript projects to experiment (in any web browser) the exact Optimal Walling and the Tile Indicator ideas I had, without the limitations of the scenario editor.
So, it will be even easier to check these ideas (but playing the scenario still presents additional QoL ideas + people not playing it would miss some Easter Eggs 
).
You are totally right, this would indeed make it easier to avoid holes in walls, which according to everyone judgement can be a positive or a negative thing.
This is one of the reason of the scenario: letting players experiment by themselves and making their own decision 
One argument to support the Tile Indicator is that there are mods which can help reducing the holes issues, but at the cost of aesthetic downgrade (this is subjective of course), see for instance SOTL video (which I used as basis for the wall with holes btw 
). For instance, grid mod, small trees, useless plants remover, short walls, simplistic terrain colors with sharp edges, terrain smoothing… can help to detect holes in walls, but with some graphics alterations.
In fact, for the grid, it was solved with the last patch and would have been one of the suggested QoL of Age of Salamanders if I posted it before the last update 