If I may give my uneducated take after reading a bit about how you feel about what defines a shield healer...

...I always saw the idea of a shield healer not in their efficiency of producing an HP barrier, but the availability of such and the frequency of how often their mitigation tools are available, especially cornerstone tools like Sacred Soil and Kerachole.

Likewise I saw pure healers as such with their availability to basic regeneration and high HP heals as well as frequency of other direct heals, primarily AOE ones.

Tbh I personally find the distinction between those two roles kinda weird. Pairing a WHM/AST with a SCH/SGE is no-brainer, SCH + SGE works very well as well because of how efficient their mitigation is (the only thing they annoy each other with is HP shield priority) and WHM + AST is a meme with a potential heal output that we would need for heal checks we will never have.