When I first moved to BC, I noticed the new customer incentive game right away and was shocked at how lenient they were. They both pretty much encouraged people to do that with their very low 3 month wait time before you qualify again. Telus has apparently reduced the amount of time you receive the discount off contract to 3 months from 6, so that's something. They've also been somewhat aggressive in getting people on the 2 year contracts by giving them free TVs, laptops, Visa cards, etc., but nothing stops you from jumping ship after that's done. I never signed the contract to get anything, and in hindsight, I should have as I've been with them for more than 2 years now and could've gotten a free TV or something out of the deal.
Having said that, from the company's perspective, saying you've signed up X new customers just sounds better to shareholders than saying you've maintained a level number of customers. Shareholders always want growth, even if it's a total BS number. In addition, from a balance sheet standpoint, it's probably easier to account for a planned temporary upfront discount that can be adjusted as market conditions warrant than an ongoing loyalty one that varies by customer and may be difficult to modify. Longtime customers also often have grandfathered plans that may effectively give them some sort of discount over current market rates. I suspect the reason for favouring new customers is more the first point than the second though. There are other ways to reward longtime customers, such as temporary promotions just for them, which provides for cost certainty. Promotions like that are difficult to quantify the success of though, and shareholders may view them as unnecessary expenses. After all, if I've stayed with them for 20 years, I probably don't need an incentive to continue doing so.
Personally, I've stayed with Telus right now because it fits my needs best right now. If I felt the two were interchangeable for my needs (and let's be honest, they are for many people), I'd probably be playing the new customer game too.