Think the answer to the question you wanted to ask is because they want to get more people to sign up. Once signed most will stick around for a few contracts.
Thanks for posting, as @B13blues has said the reward cards and incentives offered to new customers are to attract them to come and try the network.
BT Mobile as an MVNO offers a competitve range of plans and prices compared to other MVNO's such as giffgaff. As BT Mobile has evolved, it has tried to offer its customers as much as it can without increasing prices. Being with BT Mobile also has some plus points that other MVNO's don't offer. If you have a supported device, you can now have access to 4G calling (VoLTE), Wi-Fi calling and the £5 discount if you have broadband, Family Sim discount some tariffs have BT sport and some come with additonal data.
Very few mobile networks offer subsidised handsets, so normally it boils down to data. If you have BT Plus and a mobile customer, you get double data and the fastes 4G speed avalible as an added bonus. By adding plus and reducing your mobile tariff can normally save you money. Unfortunately it is a very competitve market and it all boils down to what works for each customer.
As you mentioned you are an exsisting customer it is always worth speaking to the value team when your packages are up for renewal and see what they can offer (080080030)
The deals are for new and existing customers taking the service for the first time, which means they get a welcome discount, just like you would of had when you first took the service out, seems fair to me.
It's also worth pointing out that BT have never once increased the cost of any mobile plan, other providers will increase it in line with rpi annually.
To be fair inflation would have to hit 12% to cover the difference in first year cost between what they’d offer me for a new contract and what they’d offer me to upgrade. I completely get the need to entice new customers. But why should existing customers have to leave and come back to benefit?
In addition to this, I’ve just been told by BT that customers upgrading from SIM only to mobile plans SHOULD get the reward card. Now I’m just confused.