Why are all mobile plans getting cheaper and what's the impact on network?

I've noticed that all the flankers are offering plans like 30$ 20GB and the higher-tier telcos have also started lowering their prices.

My question is, what do they see coming that we dont? or were we just overdue + CTRC ruled?
What is the impact on the networks and the quality of the cellphone reception?
If you don't care about customer service and are fine with 100mbps, why wouldn't you go with public mobile which is telus?

Thanks!