Show me any math which shows that Discipline has a stronger filler spell than Renew. PW:S only becomes stronger once a high enough threshold of over-healing is applied to Renew. This means that Holy's potential output is higher. There's no logic in what you're saying.
You want to look at facts? Okay, let's look at two Priests with identical core stats. Both with 4k SP, both with 40% haste buffed, both with 35% Holy crit. One standard 57/14 Disc, the other standard 13/58 Holy.
57/14's PW:S - 1 second execute time, shields for 8,068, glyph hits for 2,149 avg, total 10,217 per cast and 10,217 HPS
13/58's Renew - 1.07 second execute time, heals for 2,294 avg, then 13,024 over time, total 15,318 per cast and 14,316 HPS
This means if you're ever under 30% over-healing, then Renew beats PW:S easily. PoM is also stronger for Holy and CoH is also substantially stronger than PW:S. Note, this is also giving Discipline equal SP to Holy, which will not be the case in game.